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 her friend, Dex (Colin Egglesfield), who is also her best friend Darcy's (Kate Hudson) fiancé, and Rachel playfully reveals she has had a crush on him since law school. On the cab ride home, it is revealed that he has always felt the same and the two kiss. They wake up in bed together the next morning to Darcy calling both of their phones. Dex sneaks out before they can talk about what happened between them. Rachel initially believes the encounter is a one night stand, but it eventually develops into an ongoing affair. The couple soon find themselves lying to Darcy and others in order to cover up their relationship. Rachel tells Darcy that she is having an affair with another friend of hers, Ethan (John Krasinski) and an acquaintance, Marcus (Steve Howey) to cover up her sexual encounters with Dex. Ethan, who has been friends with both Rachel and Darcy since childhood, is the only one who knows what has happened between Rachel and Dex after figuring it out himself, and reluctantly agrees to keep their secret.
Meanwhile, Dex's mother (Jill Eikenberry) has been suffering from depression for some time and it seems that Dex and Darcy's upcoming wedding is the only thing keeping her sane and happy. When Dex skips out on the 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 leaves Rachel in order 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. 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 and accepts that Rachel does not reciprocate his feelings. 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 reveals to Rachel that she has been cheating on Dex with his friend Marcus. She is also pregnant with Marcus' child and says that they are very happy. Rachel gives her blessing to Darcy, who proceeds to leave only to return minutes later and ask what Dex's jacket is doing in her apartment. Darcy begins to search the apartment for Dex, who comes out, making her realise that Rachel is the other woman. Darcy 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. She reveals to Rachel that she is honestly happy as she puts a hand over her baby bump. 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.Ginnifer Goodwin as Rachel White
Kate Hudson as Darcy Rhone
John Krasinski as Ethan
Colin Egglesfield as Dexter "Dex" Thaler III
Steve Howey as Marcus
Ashley Williams as Claire
Geoff Pierson as Dexter Thaler II
Jill Eikenberry as Bridget Thaler
Peyton List as Young Darcy Rhone
Something Borrowed has received negative reviews. Review aggregate Rotten Tomatoes reports that 15% of critics have given the film a positive review based on 113 reviews, with an average score of 3.9 out of 10. The critical consensus is: "In spite of solid performances from Kate Hudson and John Krasinski, Something Borrowed is an unpleasant misfire that lives down to its title." Metacritic, which assigns a weighted mean rating out of 100 reviews from film critics, the film has a rating score of 36% based on 30 reviews.
Emily Giffin has confirmed that she has written the script for a sequel, known as Something Blue.