Emily Giffin's Something Borrowed begins with protagonist Rachel White celebrating her 30th birthday in a swank New York City restaurant with her childhood friend Darcy and others. As Rachel observes the activity around her, she reflects on an event from fifth grade involving a perpetual calendar. When Rachel and Darcy found the calendar, they looked up the days of the week on which their birthdays would fall. Darcy, always the lucky one in Rachel's eyes, would naturally have a weekend birthday while Rachel was stuck with a school-day birthday.
On a whim, the girls check to see when their thirtieth birthdays would be, and Rachel believes she has won a small victory when she finds out that her thirtieth will be on a Sunday. She imagines hiring a sitter for her perfect children and going out to an elegant restaurant with her handsome husband. As Rachel mentally returns to her actual thirtieth birthday party, she realizes how different her life is from her schoolgirl dreams. While she did accomplish leaving behind her "average" life in Indiana to pursue a law degree, she hates her job as an associate at a Manhattan law firm. She is single with no boyfriend, while Darcy flourishes in her role at a glitzy PR firm and gloats over her handsome banker fiance, Dex.
As the party goes on, Rachel becomes more sullen, and Darcy becomes more drunk, to the point that Dex must usher her home. Rachel is surprised to see Dex return a short time later, but he simply came back to collect Darcy's forgotten purse. After a few more drinks, Dex escorts Rachel out and hails her a cab but then asks her if she would like another drink. She agrees, and as she and Dex spend the evening together, their judgment lapses, and they end up at Rachel's apartment where they make love and spend the night together. A shrill telephone ring awakens both characters to reality, and they hear Darcy's panicked voice on Rachel's answering machine; she reports that Dex never came home the night before. Dex hurriedly dresses while devising a plan to tell Darcy that he and his friend Marcus stayed out all night. Rachel agrees to go along with the story but immediately feels the weight of what she has done. Thus, internal conflict between her heart and conscience begins.
As Rachel analyzes her motive for sleeping with her best friend's fiance, she wonders if she did so out of jealousy. She recounts many of the painful events from her childhood when Darcy suddenly became interested in the same boy as Rachel, or how Darcy would steal the show wherever they went together. She even provided Darcy with a place to live when Darcy first moved to New York, broke and without a job. Perhaps most bothersome to Rachel is the fact that she was friends with Dex first. She met him in law school, studied with him, and became friends with him. On a whim, Rachel even introduced Dex to Darcy because she had always claimed to herself and to others that she was not romantically interested in him. However, when she considers the events of her birthday party night, she acknowledges that she was disappointed when Dex asked for Darcy's number a week after meeting her.
To complicate matters, Dex leaves Rachel a message on her work phone asking her to talk to him about the other night. Rachel reluctantly returns his call, and Dex tells her that while what they did was a mistake and cannot happen again, he does not have regrets. Rachel finds it impossible not to think about the conversation and even more impossible to act naturally around Darcy. She fears the upcoming summer because she, Darcy, Dex, and some other friends have once again gone in on a share in the Hamptons and will be spending a lot of time together.
In the meantime, Darcy informs Rachel that Dex's friend Marcus has shown an interest in her and would like her number. Rachel agrees to go out with Marcus because he seems like a decent guy, and she hopes that the date will serve as a distraction from her problems. In Chapter Five, as Rachel prepares for her date with Marcus, she reflects on her past relationships and realizes that she not only does not have a "type" but that she had been passive and always expecting the guy to leave, which they do. When Rachel and Marcus do meet for a date, Rachel finds that she enjoys his company and decides that maybe the Hamptons will not be so bad with him there. However, when everyone arrives at the rented beach house, Rachel cannot resist studying Darcy and Dex without jealousy. She is relieved when her boss at the firm demands that she come in to work on Memorial Day. Her relief is short-lived because Dex offers to drive her to the train station and uses the opportunity to tell her that he is coming over to her place the next night. Rachel could and should refuse, but does not, and so her feelings for Dex become more complicated.
After she begins spending time more regularly with Dex, Rachel considers breaking off communication with him, but then he sends her an e-mail in which he confesses that he likes "literally everything about" her. Rachel needs a confidant so she calls her friend Ethan who lives in England and confesses everything to him. Ethan, who grew up with Rachel and Darcy, laughs when Rachel first tells him about Dex because the thought of a "goody two-shoes" betraying her best friend is too ironic for him. He then tells Rachel that he is rather proud of her because she has always allowed Darcy to run "roughshod" over her. Rachel had never thought of her relationship with Darcy in such a manner, but Ethan reminds her of Darcy's boasts that she scored five points higher than Rachel on her SATs, and of Darcy's getting accepted to Notre Dame when Rachel did not. Rachel's conversation with Ethan provides her with a smidgen of boldness, and Rachel begins to see Dex often. They keep Darcy in the dark by saying that they are working late, and Darcy believes them because she considers them both workaholics.
Eventually, Rachel cannot stand the limbo in which she finds herself, but she is unwilling to give up Dex....
(The entire section is 2464 words.)