Based on the book "Something Borrowed"
Rachel is a just turned 30 lawyer who is very attractive, but down to earth, used to be overshadowed by her larger than life best friend since childhood - Darcy. It is hinted that Darcy didn't have a normal family or childhood by how she constantly goes on about Rachel being the sister she never had and the mother she always wanted. Ethan is Rachel's best guy friend, who also has known Darcy since childhood. There is a history shown that for the 6th grade dance Ethan went with both Rachel and Darcy, presumably because Darcy couldn't let Rachel have a date when she didn't. He is annoyed with Darcy's treatment of Rachel. Darcy has spent years with Rachel always acquiescing to her way because of Darcy's overbearing attitude, to the point where 6 years earlier, Rachel pretty much handed her crush from law school, Dex, over to Darcy. This resulted in them getting together and now they are engaged.
Rachel is about to enter a very stylish nightclub with a look of trepidation. She walks in to be greeted by a surprise party being given to her by Darcy. Of course Darcy is so into herself that even though it is Rachel's birthday, she makes the night about her, doing a picture PowerPoint of her and Rachel's friendship through the years, including 6th Grade and Ethan taking them both to the dance, although Ethan is clearly looking at Rachel, and some showy images of Darcy and Dex, who Darcy credits Rachel with them getting together. Darcy starts to dance on the stage, with an embarrassed Dex coaxing her down. Part of the way through the party, Darcy has gotten excessively drunk, so Dex is leaving with her. He points out to Rachel it was kind of her to act surprised despite him knowing she wasn't. Ethan also is avoiding Claire, another mutual friend who Ethan made the grievous error of sleeping with despite not being interested in a relationship with her, and now Claire has a massive crush on Ethan. Ethan and Rachel are left after the party, discussing Claire and how Ethan likes Rachel, although he doesn't get to speak further about it as they are hailing a cab when one pulls up with Dex in it. Darcy seems to have left her $2k Purse somewhere in the club, and Rachel offers Dex to help find it.
Dex feels like Rachel didn't fully enjoy her party and suggests getting drinks together, which she accepts. As they drink there is a moment when a very attractive, Type A woman hits on Dex, which he ignores while Rachel is having a auditory memory of an incident you're not quite clear about and the woman keeps looking at Rachel with a nasty "WHY is he with YOU" kind of look, to which Dex responds to by asking the woman to stop staring as she's making his girlfriend upset. Rachel and Dex get drunk and Rachel admits she had a crush on Dex back in the day, then feels embarrassed she told him this as he is marrying her best friend. Dex tells the cab driver that they have 2 stops and gives Rachel's address and then his. Silent since Rachel's admission, Dex ends up drawing Rachel into a kiss and she responds. The cab driver laughs and says "okay, just one stop".
The next morning Rachel and Dex wake up at Rachel's house to Darcy's voice via an answering machine set on loud, whining that Dex never came home after going to look for her purse and demands Rachel call her as she needs her, despite it being 7AM the night after a party. Dex dashes out, and Rachel spends the rest of the day horrified with her actions.
Rachel gets a call from Darcy, but it's actually Dex calling from her phone, wanting to see if Rachel is okay. They don't get to talk long though as Darcy comes out and takes over the phone. Darcy goes dress shopping with Rachel and she muses over if Dex has cheated on her and then asks (or rather demands) that Darcy come to the Hamptons to stay at some house Claire's family seems to own and they have an open invite to. Darcy's plan is to hook Rachel up with Marcus, a close friend of Dex's. Ethan is coming as well, on request by Rachel.
The truck pulls up with Darcy hooting and howling. Marcus is inside and he is the polar opposite of Rachel; a loud, womanizing party-hard type of guy. The weekend commences and Rachel is miserable, Darcy is all over Dex, who's been trying to talk to Rachel about what happened. After an evening of Marcus pawing all over her, Claire attempting and failing to seduce Ethan and Darcy having extremely loud sex with Dex, Rachel heads home early, using work as an excuse.
Dex is feeling terrible, obviously having some feelings for Rachel. We learn through flashbacks that Rachel and Dex met at law school and had a connection, but both were too shy to admit it. Dex takes a chance and asks Rachel out for a celebratory dinner after finals, which she accepts. However midway through this "date", enter Darcy, who seats herself next to Dex, and instigates a truth or dare game, daring Dex to ask Rachel on a "real" date. Rachel, feeling on the spot, denies they are anything more than friends. Although Dex seems sad about this, he misunderstands Rachel not arguing him asking Darcy out on a date. So it turns out Darcy really stole Dex from Rachel years ago.
Dex fell in then out of love with Darcy, but Darcy's pushy attitude resulted in him never leaving the relationship, as it also kept him close to Rachel. After the initial incident, Rachel and Dex have an awkward few weeks before finally Dex suggests they spend a weekend together to see what their feelings are all about. They enjoy themselves immensely, however after bumping into Dex's parents, Dex's father figures things out and pulls Dex aside to explain that his mother, who it is discussed throughout the film suffers from manic depression, is doing well mentally because of Dex getting married and if Dex doesn't she'll fall apart. He also reminds him that he was raised better than to be a cheater and liar. The sad thing is the father is more concerned about the appearances, than the happiness of his son. He even offers to buy a house for Darcy and Dex to show how serious he is about wanting him to follow through, and of course Darcy is thrilled about the prospect of being purchased an expensive home.
Ethan, who has told Claire he's gay to get her to stop pursuing him, discovers the tryst and instead of scolding Rachel, commends her for once doing something she wanted after years of always letting Darcy get things. He is concerned that Dex may be stringing her along, however, and insists that she should come clean.
Dex shows up at Rachel's apartment begging to come inside. Rachel allows Darcy (who is there for an impromptu sleepover after the bachelorette party) to think it's Ethan. After Darcy tells this "secret" one weekend at the Hamptons, upsetting Claire who thinks Ethan is gay, there is a point Ethan finds keeping the secret too great a burden, and almost lets it be known, only to be stopped by Rachel whacking him in the face with a badminton racket. He is upset that Claire is upset now, and can't believe that Rachel is allowing this all to happen, as she is better than this and he cares a great deal about her and can't take it anymore. Fortunately he has taken a job in London so he can get some space from it all.
That night at a bar, Rachel argues with Dex about him not canceling the wedding and flashes back to the night Darcy butted in on their date, how she said it was okay when he chased after her, since she was used to Darcy getting what she wanted and she could tell she wanted Dex. She turns and runs back to the bar and comes clean to Dex about how she feels. Although you can tell Dex is moved by this, he refuses to cancel the wedding as she suggests because of his mother. Rachel is crushed and she shuts herself down. The wedding is coming up and she is heartbroken. Ethan has moved to London by now, so she goes to see him and lets him know what happened. Ethan says he knows what it's like to love someone and not have it work out. He has been in love with Rachel since childhood, although he realized he wasn't the one for her. Dex is, and he wishes it could work out.
During her London visit Darcy calls freaking out demanding Rachel to come back home, with Ethan writing answer suggestions like "your fiance's a douche", "I just need some time" and "I'm madly in love with Ethan". Rachel, who really does need time for herself simply, and finally, says NO and hangs up. She loves Darcy but enough already. She's willing to stand up for herself. She knows sleeping with Dex was wrong because he is with Darcy, but she refuses to feel guilty about how she feels about Dex. The movie flashes to Dex having dinner with his folks and Darcy. Mom picks up at Dex doesn't seem happy, and she's concerned. Rachel comes home and Dex is waiting for her at her place; her office told him when she'd be back. He explains he called off the wedding and how much he loves her. As Rachel processes this Darcy knocks on the door, having sneaked into the building with a pizza guy. Dex hides, not noticing he left his jacket on a couch by the door, and Rachel lets Darcy in who announces SHE cancelled the wedding, despite this not being true. It is alluded throughout the movie that some of Darcy's claims are untrue, such as being accepted to Notre Dame when Rachel, a better student, wasn't. Ethan points out this couldn't have happened as they never saw the letter of admission, and that Darcy would have shown it to them. Darcy explains Dex "said it first" but she had been thinking the same thing. Darcy declines the suggestion of going for a walk (to get away from Dex) and confronts Rachel with lying about saying she slept with Ethan and Marcus. It turns out Darcy knew Rachel lied about Marcus because Rachel claimed he was circumcised. Darcy knows he is not, because since 4th of July she's been sleeping with Marcus, and she is now pregnant by him.
Rachel is stunned. Darcy is optimistic about Marcus, feeling better having come clean. As she leaves she glances down and comes back and knocks on the door a second later, having realized Dex was there. Darcy demands to know where Dex is. He steps out and Darcy realizes Rachel is the one who was sleeping with Dex. Dex is pissed Darcy got pregnant by Marcus, but of course Darcy is focused on herself and despite having done the same thing, screams to Rachel she hates her and never wants to see her again. As she leaves she is snappy to Marcus, who had been waiting outside.
The movie flashes to a few months later. Rachel is walking home with some dry cleaning and sees Darcy, now 5 months pregnant but looking fabulous. She points out one of the shirts in Rachel's package is the one she got for Dex, saying Rachel was always jealous of her since Darcy got into Notre Dame. Rachel says she sorry she hurt Darcy, but she isn't sorry she slept with Dex, having realized what Ethan told her earlier; if the roles were switched, Darcy would have done it with no issue. Darcy assures Rachel she is happy with the prospect of motherhood. Rachel takes a phone call and walks around the corner, to meet Dex, waiting for her at a bench. They are together and truly happy, and they walk away holding hands.
Part way through the beginning of the credits we see Darcy in London, calling out to Ethan, pregnant and having gone to London to start her life over, Marcus obviously not with her and Ethan running for his life.