Rachel White (Ginnifer Goodwin) is a single attorney working in a New York law firm. After too many drinks on her 30th birthday, Rachel grabs a cab with Dex (Colin Egglesfield) and playfully reveals she has had a crush on him since law school. The problem is, Dex is also her best friend Darcy's (Kate Hudson) fiance. They wake up in bed together the next morning to Darcy calling both of their phones. Dex sneaks out and they do not have time to speak about what happened between them. Rachel initially believes the encounter is a one-night stand, but it is actually the beginning of a sincere affair once Dex tells Rachel he is in love with her. What also follows is a web of lies created by Rachel and Dex to Darcy and several others. Rachel continuously conjures up false stories of sexual relationships with Ethan and Marcus to cover up her ongoing sexual encounters with Dex.It is revealed that Dex & Rachel were in law school together & Rachel had a huge crush on Dex & told Darcy all about Dex. But just before Dex got the courage to ask Rachel out, Darcy stepped in & got Rachel to say that Dex & her were just friends. That way Dex asked Darcy out instead. Darcy always overshadowed Rachel & liked to be the center of attention. Darcy tries to push Rachel towards Dex's obnoxious friend Marcus, but Rachel is repelled by him.Ethan (John Krasinski) has been friends with Darcy and Rachel since childhood. He is the only one who knows what has happened between Rachel and Dex, which he is reluctantly keeping a secret.Meanwhile, Dex's mother (Jill Eikenberry) has been suffering from depression for some time and Dex and Darcy's wedding is the only thing keeping her sane and happy. When Dex skips out on 4th of July weekend in the Hamptons to stay in the city and spend time with Rachel, they run into his parents. His father suspects something is going on between the two of them and he tells Dex to end it as soon as possible, saying that what he wants should not be considered a priority when it conflicts with what is right. Considering his mother and father's feelings, Dex gives Rachel the cold shoulder and leaves her to go back to Darcy.As the wedding nears, Dex and Rachel are speaking less and less. At the beach, their secret is almost revealed by Ethan, who is frustrated with Rachel, especially for lying to people she cares about, but Rachel stops him by hitting him in the face with a badminton racket. This results in his nose becoming very badly broken. Ethan is frustrated with Rachel because she doesn't see that he is the only one who really cares about her. That night at the bar, Rachel finally tells Dex that she wants him to call off the wedding so they can be together, but he tells her that he can't.Ethan moves to London for a job and Rachel flies out to visit him. He reveals that he loves her and just wants to see her happy but is sad that Rachel refuses to see what's right in front of her. Rachel returns to New York to find Dex sitting on her doorstep. He reveals that he ended things with Darcy and called off the wedding.Rachel is ecstatic until Darcy ends up at her door begging to come in to talk. Dex hides and Darcy enters her apartment. She tells Rachel that she has been cheating on him with his friend Marcus. She is also pregnant with Marcus' child and says that they are very happy. Rachel gives her her blessing and Darcy proceeds to leave but only to return and ask what Dex's jacket is doing in her apartment. Dex then comes out and Darcy figures out Rachel is the other woman. She becomes infuriated and storms out of the apartment, telling Rachel that she hates her.Two months later, Rachel and Darcy run into each other on the street. They have not spoken since the incident and Darcy says that she is happy, but doesn't sound so convincing. Darcy sees that Rachel has picked up one of Dex's shirts from the dry cleaner, Rachel is unapologetic for sleeping with Dex but is sorry for hurting Darcy. As they leave, they turn back and glance at each other, with Darcy smiling and waving. Rachel walks around the corner to Dex sitting on a bench. They walk down the street together holding hands.The film ends with an ambiguous scene during the credits. Darcy shows up to surprise Ethan in London; Ethan tries to ignore her and briskly walks away. The screen turns black with text indicating that the story is to be continued, which may indicate that a movie based on Something Blue, which is the book sequel to Something Borrowed, will follow.