Laura and Dean are a recently married couple in Manhattan with two young daughters, Maya and Theo. Laura is a novelist who appears to be stuck in a rut as she struggles to finish her latest book. Meanwhile, Dean is a successful entrepreneur at a burgeoning tech start-up, surrounded by young and attractive co-workers, leaving Laura in charge of their daughters. One night, after arriving home from one of his frequent business trips, Dean climbs into bed and begins to kiss Laura passionately. However, once he recognizes her voice, he abruptly stops and goes to sleep, much to Laura’s confusion. The next day, Laura finds a woman’s toiletry bag in Dean’s luggage; he later explains that it belongs to his business associate, Fiona, and he offered to carry it in his suitcase because she could not fit it in her carry-on.
Laura decides to confide her misgivings about Dean to her father, Felix, a wealthy, semi-retired art dealer. A longtime playboy, Felix flirts with most women he encounters and believes that men are biologically wired to cheat. Convinced that Dean is having an affair, Felix proposes an investigation into him and encourages Laura to check his phone for incriminating text messages. Laura reluctantly does so, but finds nothing out of the ordinary.
While away on a business trip, Dean FaceTimes Laura on her birthday and surprises her with a gift, a Thermomix, which she does not seem particularly excited about. Despite previously insisting that she did not want to celebrate her birthday, Laura agrees to go out to a restaurant with Felix. He reveals that he had Dean followed and he was spotted shopping for jewelry at Cartier. As Laura grows suspicious, Felix picks up her in his vintage sports car one night and convinces her to stake out Dean as he attends a work dinner. After Dean leaves in a cab with Fiona, Felix speeds through the streets after them, but he and Laura end up pulled over by two police officers due to reckless driving. Felix knows the father of one of the officers, using this to charm his way out of a ticket.
Felix later discovers that Dean is planning a trip to a Mexican resort, which Laura does not believe. The next day, Dean casually tells her about the trip. Upon learning that Fiona is going on the trip as well, an increasingly suspicious Laura calls Felix, who convinces her to follow Dean to Mexico in order to spy on him. At the resort, Laura and Felix eventually spot a woman in Dean’s room one evening. When Laura rushes in to confront Dean, she is surprised to find Fiona in the room with her girlfriend. Meanwhile, Dean calls Laura to inform her that he left early and is on his way back home. Realizing her mistake, Laura lashes out at Felix, accusing him of being selfish and berating him for his poor treatment of her mother, on whom he cheated many years earlier.
Back in New York, Laura and Dean have a heart-to-heart conversation, in which they share their respective fears and insecurities: she had been feeling alienated from him due to his constant business trips, while he said he had been busy working because he wanted to be a better provider for his family. They eventually reconcile, while Laura’s adventure inspires her to successfully write a detective novel. Sometime later, Felix visits Laura and the two make amends. While at a restaurant, Dean surprises Laura with a second birthday gift—an engraved Cartier watch. Laura takes off the vintage watch Felix had previously given her and puts on Dean’s, symbolizing that Dean is the main man in her life now.