Joanna, portrayed by Keira Knightley, is a writer happily married to Michael, played by Sam Worthington, who works as a commercial real estate agent in the bustling city of New York. Their life takes a turn when they attend a party with Michael's colleagues, including Stuart and Truman. During this social gathering, Joanna observes Michael spending a lot of time with the enchanting Laura, brought to life by Eva Mendes, a colleague he had not mentioned before. The tension escalates when Joanna discovers from Andy that Michael has been traveling a lot with Laura for work, creating a seed of doubt that flowers into suspicion of infidelity.
After the party, Joanna confronts Michael about his relationship with Laura, asserting that he deliberately kept her existence from her. Michael insists he has nothing to hide, leading to an intense argument followed by a moment of reconciliation. However, Joanna cannot shake off the feeling that Michael is downplaying his attraction to Laura. This unease leads Joanna to reflect on their relationship, inciting thoughts that perhaps they married too soon.
The following morning, Michael departs for Philadelphia with Laura and Andy for a work trip. Meanwhile, Joanna goes out for coffee and unexpectedly runs into Alex, an old flame played by Guillaume Canet. Their encounter leaves Joanna feeling nostalgic, prompting them to reconnect and eventually meet up again for drinks. Throughout their evening, which later includes dinner with friends Sandra and Truman, Joanna shares that she and Alex had a romantic past, dating back to a time when she was briefly separated from Michael.
As they reminisce about their relationship, Joanna confesses that her feelings for Alex still linger, especially during tough times. In a twist of fate, they find themselves locked out of Andy's apartment, leading to an impromptu party with Sandra and Truman where their chemistry ignites and culminates in a passionate kiss in an elevator.
Later, Joanna and Alex return to his hotel room, and though they share a tender night together, she declines to take things further physically. This intimate moment is contrasted with Michael and Laura's own escapades during their business trip, highlighting Michael's moral conflict. After a series of flirtations, including a seductive conversation at a hotel bar, Michael ultimately gives in and shares a physical connection with Laura, despite previously affirming he’d never cheat on Joanna.
The next day, guilt overwhelms Michael as he finds a note from Joanna expressing her unwavering trust in him. In a rush of self-reflection, he leaves Philadelphia early, while Joanna shares a heartfelt goodbye with Alex, marking the end of their renewed bond. Upon returning home, Michael finds Joanna in tears. They tentatively attempt to reconnect, making plans as they navigate their tumultuous feelings. As they embrace, Michael’s affection seems fueled by guilt and confusion, heightened by Joanna’s choice of attire from the night before. The film draws to a close with a poignant moment, cutting to black as Joanna prepares to reveal the complexities of their relationship and the choices they've made.
