The movie opens with a voiceover by Jane (Katherine Heigl) as she explains when she first discovered her "calling" in life. The scene is from her childhood when she, her younger sister Tess and their father attend a realtive's wedding. This is shortly following the death of their mother so Jane must act as a mother figure for her sister. When her father asks Jane to take Tess to the bathroom, she comes across a distraught bride who has torn a seam on her dress leaving a whole at the back of the bodice that shows the upper half of her rear end. Jane has a moment of epiphany and uses her sister's hair ribbon and sews up the back fo the dress in a becoming way. The ecstatic bride then asks Jane to carry her train down the aisle as a reward.
Flash forward in time to the present -- Jane is in a bridal shop with seamstresses all around her hemming a wedding dress. A woman walks in and says "The bride is on the phone" where you discover that Jane is the same size as the bride and is doing her bridesmaid duty of having the wedding dress hemmed the morning of the wedding for the panicking bride. She leaves the store, dress in hand, wearing her bridesmaid dress and hurries to the nuptials making it in time for the bride to change and get ready for her ceremony.
During the ceremony, Jane continuously looks at her watch and a guest at the wedding (played by James Marsden) notices her impatience. As soon as there is a break in the festivities, Jane rushes out and hails a taxi whose driver she offers $300 for the entire night if he agrees not to look in the rearview mirror or he will suffer a deduction from that sum. Jane proceeds to change into another dress which is revealed as a traditional Indian sari that is her bridesmaid's dress for another wedding on the same night. Throughout the remainder of the night, she speeds between weddings/receptions, changing in the taxi which is seen by Kevin (James Marsden).
At the end of the night, Jane is at the first wedding preparing for the bouquet toss which comes straight for her until she is tackled by some over-enthusiastic girls and knocked unconcscious. When she wakes up, Kevin (James Marsden) is leaning over her and helps her up, where she swoons, and then offers to accompany her home.
On the trip to Jane's apartment, Kevin comments on her two-wedding night and proceeds to mention his opinion about the ludicrousness of the ceremony. This sparks a lively debate between the two about the sacredness of a wedding and marriage which leaves Kevin charmed and Jane annoyed. Once they reach Jane's apartment building, she leaves and Kevin discovers her "filofax", or day planner, in the taxi and decides to look through it.
Kevin Doyle (James Marsden) discovers that Jane has been in 7 weddings that year and, as he is the writer of the local newpaper's "Commitments" section (under his full name Malcolm Doyle) he asks his editor if he could write a feature story on Jane (Heigl) for the front page of the section in the hopes that, if it is good enough, he can get out of the "taffeta ghetto."
The next scene, we see Jane going to the office with her friend, Casey (Judy Greer), who was also in one of the weddings with her. Once inside the office, Jane is heckled for her longtime crush on her boss, George (Edward Burns), to whom she is a personal assistant. Everyone knows about her infatuation except for George, who owns the eco-friendly outdoor sports magazine business that they work for. Flowers are delivered for Jane without a note and she excitedly assumes that they are from George. When she leaves work with Casey (Judy Greer), Jane says that she has to go and pick up her sister from the airport but that she will see Casey later at another coworker's engagement party that night.
At the airport, Jane picks up her sister, Tess (Malin Akerman) who has been working in Milan as a model. They go back to Jane's apartment, where Tess will be staying for a few weeks, and they talk about their parent's wedding photo which Jane has framed in her house. They agree that it was romantic but Tess dislikes her mother's dress while Jane is completely taken with the entire event -- at the boat house, at sunset, etc. Tess proceeds to pick up a pile of newspaper clippings on the counter that are featured engagement profiles from the paper written by her favorite writer of engagement and wedding announcements, Malcolm Doyle, whose sweet descriptions and feelings make his the best. As they talk, Jane invites her sister to the engagement party and Tess admits that she has plans to meet with some friends from Milan for drinks, but will try and make it.
The engagement party takes place in a bar and has a gothic theme. Jane is talking with Casey when George shows up and asks Jane what he had purchased the couple (which she had obvioulsy done for him.) After a few minutes of small talk, George asks Jane if "what he put on her desk" that morning was okay since their relationship had not "been that way" before. Jane says it was okay and, once George walks away, Casey (shocked that the flowers were actually from George) demands that she walk up to George and express her true feelings. Confident, Jane begins to walk toward the bar, and George, when she notices his gaze locking on something across the room. George is captivated by Tess (Akerman) who has just entered the bar looking for her sister. The two lock eyes and walk towards each other, directly in front of the deflating Jane. As they are introduced, George mentions that Jane is his lifeline and that she had even picked up his dry cleaning which he had left on her desk that morning. George then asks Tess if she would like to get a drink at the bar, an offer that Jane had turned down from him earlier in order to seem responsible.
As Tess (Akerman) and George (Burns) head to the bar, Kevin (Marsden) appears and asks Jane (Heigl) if she liked the flowers that he sent. Upset at the situation, Jane excuses herself to scream in an adjacent room which turns out to be a private anniversary party where she embarrasses herself with her outburst. When she returns to the dance floor where Kevin is waiting, holding her drink, he gives her filofax back to her and jibes her about her full-time wedding duties and asks her to get a drink. Depressed about the situation, Jane declines and goes to the bar to influence her sister to leave for the night. However, Tess asks George to go dancing instead and, though George invites Jane, Tess subtly hints at her sister to decline the offer and they leave together.
Jane goes back to her apartment but cannot sleep while her sister is out until the early hours of the morning. So, frustrated, Jane gets out of bed and does sit-ups, reads a wedding magazine, brushes her teeth and ultimately begins scrubbing the stove in the kitchen before Tess returns. Trying to play-down her obvious agitation, Jane listens as her sister talks about her "wonderful" evening and asks for details about George. Jane obliges, and gives unique details about him that reveal her feelings for him, but Tess does not pick up on her sister's crush. Tess reveals that she and George had gotten kind of intimate and that he wanted to have lunch the next day. She asks her sister if this would be alright and Jane, always sacrificing for others, says that it is fine.
The next day, the girls visit their father at his hardware store. While they are having breakfast in his loft upstairs, George calls Tess and she giggles with him on the phone in another room while Jane becomes upset.
In the office, Jane looks over into George's office and sees her sister in a short dress leaning over the desk and flirting with George. Jane opens her filofax and sees that Kevin has written his name on every Saturday in her planner along with his phone number. She calls him and tells him to leave her alone, but he renews the offer for drinks saying if she ever wanted to hang out with someone who didn't need her to "take them to a fitting or a cake tasting" he would be available. Once Jane hangs up, Pedro (David Castro), George's "little brother" from the Big Brother program, arrives for George to take him to his baseball game. George insists that both Jane and Tess come along so they leave for the game.
At the game, George, Tess and Jane go to the concession stand to order some refreshments before the game. Jane orders a chili dog and asks Tess if she would like one when Tess says that she is a "recent vegetarian". Jane scoffs as she knows that Tess is lying because of Jane's statement that George does not eat meat. Tess also proceeds to comment on her hiking trips (while Jane rolls her eyes) and her love for their dog "Tory". Jane, finally fed up with her shenanigans, corrects her sister that the dog's name was "Toby" which Tess explains away as being what she called the dog due to her lisp. George rejoins and says that he, too, had a speech impediment as a child--he stuttered.
Pedro calls for George to come and "shag some flies" with him on the field and when the girls are left alone, Jane confronts her sister on lying and hating Toby. Tess justifies herself saying that she could "learn to not eat meat", she likes the outdoors for tanning and she only hated it when Toby slobbered on her. George saves her from her sister's irritation when he calls for Tess to try her hand at batting (in a dress and heels).
The next few scenes are a montage of Tess and George's relationship as Jane walks in on them making out in the apartment, sees a slideshow of pictures of them on George's office computer, etc. One night in the office, after everyone else has gone home, Jane goes into George's office to straighten up and notices that he has left his wallet. Scanning his desk calendar, she hurries to the restaurant listed to give him his wallet. When she walks into the restaurant, she sees that they entire area has been rented out, there is a band and they drop a banner that says "Will You Marry Me". George comes in and says "No, guys, that's not her" and they roll the banner back up. Jane, crushed, gives him back his wallet just as Tess walks in. The whole scene replays as George has his dog, Gatsby, bring in the ring, and he proposes to Tess while Jane tries, in vain, to escape the scene.
After having to bear witness to the man she loves proposing to her sister, Jane agrees to meet Kevin (Marsden) at a bar. She explains her being upset as having to do with the anticipation of being "dumped on" by her sister to work out all of the wedding details. Kevin chides her for not being able to say no to anyone and they talk about her constant need to do whatever anyone asks.
Then, the girls are back at their father's apartment with George and their father says that George is his "hero" for giving Jane a job and marrying Tess since she has now decided to stay in town. Their father hurries from the room to retrieve something and comes back with their mother's wedding gown for Tess. Jane is mortified and heartbroken as Tess picks up the dress saying how honored she is to be able to wear it (regardless of her earlier comments on how she disliked the dress). Noticing Jane's face, she says "I know that you wanted to wear mom's dress first..." to which Jane replies "It's fine--it's what she would have wanted."
In the next scene, Jane is walking with her sister as her sister asks her about countless details that Jane has been taking care of alone. As they walk, Tess mentions that she and George are getting married in the same boathouse as their parents, in 3 weeks (that was the only opening and she didn't really want to wait, anyway). Jane is visibly upset since that was her plan for her wedding, but she defers again. Tess, aware that she has no real friends, asks Jane if Casey would be a bridesmaid because "she's pretty and won't throw off the asthetics".
Jane, Tess and George visit a custom cake bakery and Jane uses her previous wedding business with the baker to get him to agree to make the wedding cake in 3 weeks. As George walks away to take a call, Kevin (Marsden) walks in and introduces himself as Malcolm Doyle, the Commitments writer who had contacted them about writing a feature on their wedding. Jane is stunned by this revelation and accuses him of lying to her, which he denies. Tess tells Kevin that Jane is his number one fan and that she saves his articles which only embarrasses Jane (and entertains Kevin) more. Jane pulls him aside and asks if he is really as cynical as he had acted in the past about weddings and he says yes which makes Jane feel like she "just found out her favorite song was written about a sandwich."
In the next scene, Kevin arrives at Jane's apartment where she is doing dishes alone. Jane refuses to let him in but he insists that he needs to talk to all of the bride's friends and family for the article and gets her to open the door for "Tess and George." Kevin proceeds to turn on his tape recorder and ask a few questions about Jane's feelings on the matter (which she gives begrudgingly) when he notices a hall closet that is bursting open with a variety of colorful fabrics. When he asks Jane what is in the closet, she runs to close the door but Kevin manages to pry them open to reveal every bridesmaid dress and accessory that Jane has ever been asked to wear. Kevin insists that they are all hideous and Jane picks out one that she thinks is becoming and goes to her room to try it on. When she shows Kevin, he snaps a picture of her and she ultimately agrees that it's not that becoming. Entertained, Jane proceeds to put on every bridesmaid dress as a sort of informal fashion show for Kevin (who takes a picture of each outfit) including such horrible themes dresses as "Gone With the Wind" and a full SCUBA outfit for an underwater ceremony.
Once all of the dresses have been shown, Kevin counts the photos (27 different dresses) and asks why she keeps them and suffers through them. Jane says that she has had some good times in those dresses and she wears them because one day those people will be there for her on her wedding. Saddened by the thought of her own wedding, Jane asks to just finish the interview and puts away the dresses.
Back at the office, Kevin's editor asks for the article which he has not yet finished. He sends her his working draft and she loves it, guarenteeing him a spot on that Sunday's front page of the section. Kevin begs that it be postponed for a week as he tries to delve deeper into Jane's person with the editor commmenting "if I didn't know you better I would think that you have developed a little crush". Kevin thanks her for finally conceding and agreeing to postpone the feature.
Jane arrives at George's apartment looking for her sister to finalize the guest list but Tess won't let her in. When she barges in, she finds that Tess has hired Pedro to clean the apartment which would infuriate George so Pedro has agreed to keep it a secret as he wants the afterschool job.
Jane and Kevin are in a store registering Tess for her wedding gifts. As they scan items, Kevin comments on Tess' outrageous abuse of her sister's services and mocks Jane's comments on the meaning of the gifts ("and this is the vase that Tess will put the flowers that George brings home just because"). As Kevin makes her progressively more angry with his cynicism, Jane says that she believes that he hates weddings because he hasn't found someone to love or he was dumped at the altar. Kevin says "bingo" as his wife had run away with his roommate from college after their big ceremony. Jane, feeling horrible about being right that his wedding had ended so poorly and made him cynical, asks if he would like to find all the ugliest things in the store and "register Tess for them."
Jane receives a call from George to come preview the menu for the wedding with him since Tess would be getting her hair done and unable to attend. Jane happily joins and almost tells George of Tess' lies but decides against it at the last minute. Kevin calls Tess asking for Jane, and Tess tells him where she is (at the boathouse). Kevin shows up and sees Jane laughing with George and instantly realizes her feelings for him. He tries to sneak away but George sees him and Kevin volunteers to go with Jane to pick up vintage table linens in another city since George has to make dinner with his parents (Jane tries to get George to come instead, but Kevin is insisitent.)
In the car driving in the rain, Kevin confronts Jane with her "sick" love triangle--helping her sister plan her wedding to the man she is secretly in love with. Jane becomes angry at Kevin's comments and begins to drive haphazardly. Kevin begins to get nervous at their speed and warns that they might hydroplane, only annoying Jane more. Of course, they hydroplance off of a curve, down an incline and into a tree where there car is stuck in the mud. They run in the rain to a nearby bar and find that the phones are out and they can't get cell phone reception. Kevin, deciding that there is nothing left to do but wait out the storm, sits at the bar and orders a drink. Jane joins him, defeated, and insists that she will only drink one.
Next scene, and presumably many drinks later, Jane asks Kevin about a specific wedding that he had written about where the ceremony was held on the anniversary of the parents' deaths and the brother was flown in from Afghanistan for it. Kevin claims he doesn't remember it and Jane insists that he must and he's doing it all to appear "mysterious and sexy". Kevin does tell her that his favorite part of weddings is watching the groom's face while the bride comes down the aisle (which Jane had earlier recounted to George as her favorite part). Suddenly, the juke box comes on playing Elton John's "Bennie and the Jets". Kevin begins to sing with the juke box but Jane laughingly points out that he is not singing any of the right words, so she tries (only further butchering the lyrics). Suddenly, they are both standing on the bar in front of a crowd of people dancing and singing the chorus of the song at the top of their lungs. At the end of the song, the frenzied crowd helps Kevin down, who turns to help Jane off of the bar. When she comes down in his arms he admits that he "cried like a baby" at the wedding she had mentioned and they start to kiss.
Next, you see them back in the car as Jane works to take Kevin's shirt off and they fall into the back seat.
The next morning, Kevin brings coffee to the car and they both walk to a nearby diner for breakfast while they wait for the towtruck. At breakfast, one of the people from the night before point them out as "Bennie and the Jets" and they laugh about the night before. The waitress comes to give them their orders and she notices Jane and says, "Hey, you're that girl!" Jane nods and says, "Yes, I'm Bennie, he's the Jets" with a smile but the waitress rushes to the counter and brings back a newspaper. Jane takes the paper and sees her pictures all of the feature story about her 27 bridesmaid experiences. Kevin suddenly realizes that his editor ran the piece early and tries to explain. Jane, upset, leaves the diner and slaps Kevin soundly in the face for using her.
Back at the paper, Kevin demands to know why his editor ran the piece early since he had not even had time to tell Jane about the piece yet. The editor insists that he should be thanking her, and walks away.
Jane arrives back at her apartment where her sister is fuming over the article which portrayed her as "Bridezilla". She yells at Jane and answers the phone, which turns out to be Kevin calling, and she screams at him. She then storms out of the apartment yelling, "You might want to alert traffic control because Bridezilla's on the loose!"
The next day at work, Jane throws away a stack of missed call notices from Kevin and Casey comes over to talk. Jane asks that they not talk about the article when George calls her into his office. She immediately begins apologizing, but George insists that it is alright and he only wanted to make sure that she was okay. Grateful, Jane nods and George says, "No one reads that section anyway" even though it is the most popular section of the paper (as mentioned earlier in the movie).
Jane arrives at a bridal boutique where Tess is standing in her underwear with a checklist. Tess says that she has decided to forgive Jane. But insists that at that night's rehearsal dinner, Jane read off of a script that Tess wrote to go along with a powerpoint. Jane agrees and the store's employees come in with Tess' wedding dress. Tess puts it on and it is a completely different dress than their mother's. Confused, Jane asks where their mother's dress is and Tess explains that she had it incorporated into the new dress and the remaining pieces of their mother's dress have been saved for Jane. Enraged, Jane demands that Tess tell George the truth about everything or she will. Tess says that Jane would never hurt her sister like that and Jane replies that Tess is no longer her sister she's just the girl who broke her heart and cut up her mother's wedding dress.
The next scene, Jane comes into the rehearsal dinner and Tess hurriedly checks to make sure that Jane is still going to read only from the script. Jane nods and then goes to cue-up the powerpoint for the dinner. As everyone sits, Jane begins the powerpoint that she put together reading only the script, however, the powerpoint has pictures of Tess eating a large plate of barbeque with her engagement ring on, picking a cat up awkwardly by the legs, acting afraid of Gatsby (George's dog), etc. At the end of the show, Tess tries to explain as Casey pulls Pedro up to the front to try and save the dinner. Pedro, however, talks about how great Tess is for agreeing to help him start his cleaning business and letting him clean George's apartment. George leaves the room and breaks off the engagement while Jane feels horrible about what she has done.
Going outside to escape the chaos she created, Jane tries not to cry as Kevin, who had been lurking in the back of the rehearsal dinner, comes outside to comfort her. He apologizes for hurting her and said that he came to ask forgiveness and to be there for her, which was something that he had not wanted to do for a long time. He gives Jane a palm pilot to replace her outdated filofax and then walks away, saying that he would never bother her again.
Jane is in her father's store telling him that Tess will never forgive her as Tess comes in, lured by her father. The two girls have a heated argument over the situation where Tess accuses Jane of always feeling like she had to fill in as Tess' mother and Jane accuses Tess of always having everything. Tess admits to wanting to be more like Jane and that she was in town because she had been fired in Milan and her Italian boyfriend had dumped her. She had seen George as a way to start over and had jumped at the chance. Finally coming to an understanding, the two girls hug and try to decide what they are going to do now.
Later that night, Jane receives a call from George asking her to fill in as his date for a business function since his recent break-up has left him without time to get a date. She shows up to the office and George, who is grateful that everything came out since the entire affair had been so rushed, asks Jane to pull up his presentation before they go since he can't find it. When she agrees and hurries to his computer, George says, "That's what I love about you, Jane, you always say yes." Suddenly, Jane stands up and looks at George with realization. She quits and explains to him that the only reason she had stayed for so long was because she had become comfortable and thought she was in love with him. George silences her with a kiss that Jane actually says has no spark. They try again but the kiss still falls short of her expectations. Just as they are standing there, her new palm pilot rings (it's a phone too) and the ringtone is "Bennie and the Jets". Smiling widely, she apologizes to George and runs from the office.
Jane goes to the newspaper looking for Kevin and finds out that he is covering his last wedding (having been promoted out of Commitments due to the phenomenal response to his feature article). Jane hurries to the pier where the wedding ship is getting ready to leave. She jumps onto the ship and walks through the crowd looking for Kevin. As she searches, the bride notices her and knows her from the article and is excited that she is at her wedding (as though she is a celebrity). Jane quickly fills the eager bride in on the situation and the bride pulls her to the stage and gives her a microphone where she is spolighted for all of the guests to see. She confesses her feelings for Kevin who is watching in shock from the middle of the crowd. When she finishes, she awkwardly leaves the stage and tries to make her way to the back of the boat where Kevin stops her and they kiss.
The next, and final scene, is Kevin and Jane's beach wedding. Tess is welcoming guests where George and Pedro arrive. Tess introduces herself like they had never met and says she "has a hamburger a day" and her "idea of a pet is a rock". They laugh together and George sits down. The wedding begins and Jane joins Kevin at the altar with a voice over saying that he was looking at her just as she'd always hoped her groom would. Kevin asks if it was as good as she had hoped for to which she replies, "No more, much more."
The camera pans out to show that after Tess and Casey, 27 brides stand along the boardwalk each wearing the bridesmaid's dress from their respective weddings.