Plot
The free-spirited Darcy throws her friend Rachel a surprise 30th birthday. Rachel is a single attorney in New York City, whereas Darcy is engaged to a man named Dex. Rachel's close friend Ethan is her confidant.
Darcy gets drunk at the party, so Dex takes her home but returns for her Chanel purse. Rachel offers to help look for it, and he gets her a drink for her birthday. Afterwards, a drunk Rachel mentions the crush she had on him in law school. Sharing a cab, she apologizes, he kisses her, and they wake up in bed together to Darcy's frantic message that Dex never came home.
Flashbacks show Rachel and Dex in law school, growing closer. One evening, over drinks, they are sharing personal stories, but Darcy showing up breaks the romantic mood. After being teased by Darcy about Dex, Rachel says he is just a friend. He is disappointed, but she does not notice. Darcy relentlessly flirts with Dex, so Rachel leaves.
Back in the present, things between Rachel and Dex remain awkward as the wedding plans proceed and Darcy hosts frequent gatherings in the Hamptons. Rachel says she is having sex with other men (first Marcus, then Ethan). Darcy tells Rachel she cheated on Dex once with someone from work.
Ethan is frustrated at Rachel for never standing up for herself against Darcy. Friends with Rachel and sometimes foe to Darcy, he reluctantly agrees to tolerate Rachel's lies. However, he soon tires of the charade, trying to tell the group about Dex and Rachel, but she hits him with a racket, angering him for not confessing.
Meanwhile, Dex's mother is depressed, but her depression is being kept in check by her happiness with the upcoming wedding. Months after first being secretly intimate, Dex tries to talk to Rachel about what is between them; they skip Darcy's July 4 weekend in the Hamptons to stay in the city.
Running into his parents, Dex's father tells him to end it, as his wants are less important than the overall right. He stuck by Dex’s mom with her troubles, so Dex should not abandon Darcy so close to the wedding.
As the wedding nears, Dex and Rachel speak less and less. At the beach, Ethan almost exposes their secret again. That night at the Hamptons bar, Rachel asks Dex to call off the wedding so they can be together, but he says he cannot.
Ethan moves to London for work, so Rachel goes to visit him a week before the wedding. He confesses he loves her, but accepts it is not mutual. Deciding to go to the New York wedding, she acknowledges she has to support Darcy. She finds Dex on her doorstep on her return to New York, as he called it off.
Rachel is ecstatic until Darcy arrives to talk. Dex hides, overhearing her confessing that she has been cheating with Marcus (while Dex was cheating with Rachel). She is now pregnant with Marcus' child, and says they are happy. On her way out, she sees Dex's jacket and searches the apartment for him. When he reveals himself, she realizes her best friend is the other woman. Dex and Darcy argue, and she yells at Rachel through tears that she hates her and never wants to speak to her again, storming out.
Two months later, Rachel and Darcy run into each other. Darcy acts excited about her pregnancy, saying this is the happiest she has ever been. When Darcy notices Rachel has picked up one of Dex's shirts from the dry cleaner, she sees they are still together.
As they begin to walk away, Darcy turns and says that she truly is happy. Rachel smiles and says she is glad. Dex then calls, who is waiting on a bench around the corner for her. Joining him with a smile, he takes her hand as they walk together down the street.
In a mid-credits scene, Darcy surprises Ethan in London; he tries to ignore her, briskly sneaking away.
