Something Borrowed recap

At her thirtieth birthday, cautious lawyer Rachel White sleeps with Dex Thaler, the fiancé of her lifelong best friend Darcy Rhone. Rachel has loved Dex since law school but introduced him to Darcy and habitually yielded to her more forceful friend. What seems like one betrayal becomes a continuing affair after Dex admits that he shares Rachel's feelings. Through a summer of group weekends, Rachel and Dex meet secretly while Darcy plans the wedding and Rachel uses dates with Dex's friend Marcus as cover. Confidants Ethan and Hillary push Rachel to stop accepting indefinite promises, and she demands that Dex choose. He eventually leaves Darcy and comes to Rachel. Darcy then reveals that she has been sleeping with Marcus and is pregnant, only to discover Dex in Rachel's apartment and understand their affair. The wedding and the women's friendship end. Rachel and Dex stay together; Darcy's relationship with Marcus fails, and she prepares to leave for London without reconciling with Rachel.

New York lawyer Rachel White turns thirty at a party organized by her lifelong best friend, Darcy Rhone. Rachel has long been the cautious counterpart to Darcy's confidence and has often let Darcy take the more desirable role in their friendship. That history includes Dex Thaler, Rachel's law-school friend: Rachel was attracted to him but introduced him to Darcy, and he is now Darcy's fiancé. After Darcy leaves the birthday celebration, Rachel and Dex continue drinking, share a taxi, and sleep together. The next morning Rachel is ashamed and expects them to treat the night as an isolated mistake. Instead, the encounter forces her to admit that her feelings for Dex never disappeared and that her loyalty to Darcy is entangled with years of resentment.

Rachel and Dex speak again and acknowledge that the birthday night was not emotionally meaningless. Dex says that he has feelings for Rachel and doubts his coming marriage, but he does not immediately end the engagement. Their contact becomes an affair conducted around Darcy's wedding plans and the assumptions of their shared friends. Rachel confides in Ethan, an old friend living in London who understands the unequal pattern between Rachel and Darcy and refuses to let Rachel describe herself only as a passive participant. Rachel's colleague Hillary also becomes an ally. Both recognize that Dex's uncertainty leaves Rachel carrying secrecy and guilt while he retains his public future with Darcy.

The friends begin their summer routine, including weekends away together, which puts Rachel beside Darcy and Dex while the affair continues. Rachel dates Dex's friend Marcus, partly as camouflage and partly in an effort to test a simpler alternative, but Marcus is casual and does not offer the bond she wants. Dex finds private time with Rachel and continues to say that his relationship with Darcy is wrong, yet the wedding remains scheduled. Darcy talks freely about the ceremony and her future, unaware that her best friend and fiancé are withholding the central fact of their lives. The strain makes Rachel reconsider memories in which Darcy competed with her or claimed her opportunities, though those grievances do not erase Rachel's responsibility for the present betrayal.

Rachel's secrecy spreads into the rest of the social circle as she tries to explain absences, meetings, and her apparent romantic situation without exposing Dex. Claire's interest in Ethan adds pressure to the cover stories, while Ethan and Hillary continue to tell Rachel that waiting indefinitely is not a decision. A visit to Indiana and contact with childhood friend Annalise remind Rachel how long Darcy has defined the terms of their friendship. Rachel also sees that she has used Darcy's selfishness to justify avoiding a direct choice of her own. Dex remains divided between the expectations around his engagement and the relationship he says he wants with Rachel, leaving the two women to plan incompatible futures around him.

As the wedding approaches, Rachel can no longer accept Dex's assurances without action. She requires him to choose and prepares herself for the likelihood that he will marry Darcy. The affair stops feeling like a private exception and becomes a decision about the life each of them will publicly live. Rachel considers the loss of Darcy, the judgment of their friends, and the possibility that Dex's hesitation reveals a permanent weakness. Dex at last breaks the engagement and comes to Rachel, showing that he has chosen her. They are together in Rachel's apartment but have not yet managed the disclosure to Darcy or the consequences for the wider group.

Darcy unexpectedly arrives at Rachel's apartment with a confession of her own: she has been sleeping with Marcus and is pregnant by him. She expects the news to explain the end of her engagement, but then discovers Dex with Rachel and realizes that their relationship has been continuing behind her back. The symmetry does not produce forgiveness. Darcy sees the affair as a betrayal by the two people closest to her, and her friendship with Rachel ends in anger. The wedding is canceled, while Rachel and Dex remain together and begin a relationship that no longer depends on secrecy. Darcy's intended future with Marcus soon falls apart. Later Rachel encounters her visibly pregnant former friend as Darcy prepares to leave for London. They do not reconcile, but the meeting is less explosive than their confrontation. Rachel accepts both her chosen life with Dex and the lasting cost of ending the old pattern with Darcy.

Dex finally ends his engagement and goes to Rachel, choosing their relationship rather than proceeding with the wedding. Before Rachel and Dex can decide how to tell Darcy, Darcy arrives and confesses that she has also been unfaithful: she has been sleeping with Marcus and is pregnant by him. The disclosure initially seems to release Rachel from her guilt, but Darcy discovers that Dex is in Rachel's apartment and realizes the affair has involved her fiancé and her oldest friend. The confrontation ends Rachel and Darcy's friendship, and the wedding is canceled. Rachel and Dex remain together, now without secrecy but aware of the hurt surrounding their beginning. Darcy's plan to build a future with Marcus does not hold. Later, visibly pregnant and preparing to leave for London, she encounters Rachel. Their exchange is restrained rather than reconciliatory. Rachel closes the story having made a choice for herself, while accepting that gaining Dex has cost her the friendship that defined much of her earlier life.