The movie begins with a montage of shots of wedding items from a scrapbook box then a voice over from Candice Bergen tells the story of two girls (Emma and Liv) who went to the Plaza Hotel with their mothers for lunch in June and saw a wedding. They then fell in love with the idea of having their weddings in June at the Plaza when they got older. One of the things they picked up at the wedding was a hair barrette, which was dropped by the bride. Emma and Liv started a scrapbook box of wedding items including the barrette and even practiced getting married in their attic with Liv always acting as the bride. It then moves to present day where they are both 26.
Liv (Kate Hudson) and Emma (Anne Hathaway) are shown at a wedding of a friend with their respective boyfriends Daniel (Steve Howey) and Fletcher (Chris Pratt). At the wedding they critique it and say how they would change it and then go after the bouquet and a screen shot shows their intensity of trying to get it. Candice Bergen continues to voice over about their behavior.
Liv is a up and coming tough Associate lawyer who is used to getting her way and Emma is a teacher who works with a colleague, Deb (Kristen Johnston) who continually mooches her time while doing very little. That night, while Liv and Emma are going through clothes in Liv's closet they discover a Tiffany ring box, which Liv is very excited about in thinking it's an engagement ring from Daniel. Emma keeps her from opening it, and suggests she not tell anyone yet and that Daniel should see her when she opens the ring. While at the bar that night, Liv has told everyone she is engaged, including her brother, Nate (Brian Greenberg). While celebrating, Liv becomes upset and goes outside where Emma comforts her in the fact her parents are not there to see how her life has turned out.
Emma and Fletcher are eating Chinese for dinner when they open their fortune cookies, where she finds an engagement ring and says yes to his proposal. She calls Liv who is happy, but also seems a bit upset she was not first in getting engaged. The next day they are running in the park and Emma says she already has an appointment the next day with premiere wedding planner Marion St Claire (Candice Bergen) as they just can't work with anyone but her. Liv then rushes to Daniel's office and asks about the ring and if they are going to get engaged. He takes the ring out of his pocket and talks about how she is always one step ahead of everyone and if she had just waited for that evening he was about to propose, so they get engaged.
The next morning Liv and Emma go to Marion's office and ask about June dates at the plaza. Marion says there are three spots left, 2 on the 6th and one on the 27th. Liv takes the 6th, as it was her parents anniversary. As they are leaving, they tell the incoming bride that there is only one date left in June at the Plaza. While out dress shopping, Emma finds a Vera Wang dress which Liv then tries on and chooses. Emma has decided to wear her mother's dress. While shopping, Marion calls to tell them their weddings were booked on the same date, June 6th. They ask about switching with the other bride and then get her name from the secretary (who was fired for screwing up). They chase the bride around a department store while she is scanning for wedding gifts. Being unsuccessful, they then start to talk about who has to move their date and neither wants to compromise. Their fiancÚs suggest a double wedding and they don't want to share the date. As tensions begin between the two, they agree to not make any decisions until it is figured out. While Liv and Daniel are out looking at invitations, Daniel talks to Fletcher. When Emma asks about Liv, Fletcher comments they were out doing their save the date cards. Emma gets upset and sends out a mass e-mail, which is poorly typed, to her entire address book, except for Liv, telling them all to save the date of June 6th. Liv and Emma are being thrown a joint bridal shower and get into an argument over Emma's e-mail and their behavior.
They both continue with their wedding planning by making deposits on their date at the plaza, ordering invitations, picking out cakes, and not speaking to each other. Their friends refuse to take sides, so they have to find maid of honors and they both start calling really old friends and cousins with no luck. Liv hires her male assistant to be her maid of honor and Emma reluctantly asks Deb who says she will then need more help at school if she is going to be her maid of honor.
Liv and Emma then begin a series of pranks on each other. Liv steals Emma's dream DJ for her wedding, darkens Emma's spray tanning to make her orange, she sends an alternative dance instructor to their wedding dance lesson who runs them ragged and plans to tamper with their video at the wedding by showing one from their spring break trip. Emma sends Liv cookies and chocolate supposedly from Daniel so she will gain weight and not fit her dress, puts an unflattering photo in their hometown paper about their wedding and changes Liv's hair highlights to blue. They both continue to drive their fiancÚs nuts with their intensity and fighting. Emma and Fletcher seem to have the most conflict.
Liv is attending her bachelorette party at a strip club with three friends when Emma shows up and steals the show with a dance off and swinging on a rope. Liv is upset to lose to Emma and drinks several shots. The next day she is very late to work and shows up with blue hair to present a brief which she has fallen behind on, her boss is unimpressed and she is removed from the case. Emma meets Nate while he is heading to get his tuxedo and he makes a comment to her that he waited around too long.
Emma and Liv seem to be missing each other, as they don't have a friend to talk to. Liv sees Emma running at the park, but she can't hear her over her ipod. Emma calls Liv at work, but she does not get through to her, as her assistant can't hear her due to her office sign being moved.
The big day arrives and Emma and Liv are getting ready and Emma's dad visits Liv to wish her well and tells her that her parents would be proud. Liv feels bad and decides to switch Emma's movie back to what it should be, instead of the spring break she had substituted. She asks her assistant to switch it back, but he drops the CD in the flowers on purpose so it does not get changed. Emma's Dad comes to see Emma and gives her the scrapbook box they had as girls. He takes out the barrette and puts it in her hair, but Emma says it should have been Liv's. Both girls want to see each other, but it is time for them to walk down the isle. Their eyes meet in the hallway as they are about to enter their rooms and they smile slightly. As Emma enters her ballroom, the video plays of her on spring break. She gets really mad and runs into Liv's wedding and tackles her from behind. They flight and then say they can't do this anymore. As Emma gets up, Fletcher is very mad at her for ruining their wedding and he insults Liv. Emma stands up for Liv and says she wishes she were more like Liv. Emma tells Fletcher maybe they should not get married as he fell in love with who she was 10 years ago. They break up, and he leaves. Emma then takes the barrette of her hair and puts it in Livs; she then walks Liv down the isle. At their reception, Nate asks Emma to dance.
It then flashes ahead to one year later, and Emma meets Liv for breakfast. Emma has returned from her honeymoon with Nate. Liv suggests that they toast, but Emma says she is not drinking and Liv says she is not either. They are both pregnant and due on March 3.