Helen Quilley, portrayed by Gwyneth Paltrow, faces a series of life-altering challenges after losing her job in public relations. She is fired by her boss, Paul (Kevin McNally), following a night of heavy drinking that ultimately leaves her with nothing to offer clients. As she exits the building, she drops her earring in the lift, where a kind stranger picks it up for her.
In a twist of fate, Helen attempts to catch a train on the London Underground but initially misses it. The narrative then rewinds, offering an alternate storyline where she successfully boards the train. On this journey, she sits next to James, played by John Hannah, who engages her in conversation about the Beatles and how little people interact in public transit. That evening, Helen returns home only to find her boyfriend, Gerry (John Lynch), in bed with Lydia (Jeanne Tripplehorn), his American ex-girlfriend. Devastated, she ends her relationship with Gerry and moves in with her friend Anna (Zara Turner), opting for a transformative new hairstyle to signify her fresh start.
As fate would have it, James reappears in Helen's life at a bar where she attempts to drown her sorrows. He manages to uplift her spirits while also respecting her need for space. Their serendipitous encounters continue, with James encouraging her to start her own PR firm. Anna thinks James is charming, and soon enough, Helen begins dating him. Despite her hesitations following her painful breakup with Gerry, her feelings for James deepen, and they share an intimate night together.
Helen employs her PR skills to promote a restaurant owned by one of James's close friends, leading to remarkable success. Meanwhile, Gerry reenters her life, expressing his desire to rekindle their relationship, even as Lydia complicates matters by revealing her pregnancy with James's child. Upon uncovering Gerry's infidelity once more, Helen decides to leave him for good.
Then, Helen discovers she is pregnant and, believing it to be James's child, seeks him out at work. To her dismay, she learns from his secretary that he is still married to Claudia (Nina Young). When they meet on a bridge, James clarifies that although he is separated, he still maintains the facade of a happy marriage due to his mother’s condition. After they confess their love for one another, tragedy strikes as Helen is hit by a van.
In the alternate storyline where she misses her train, Helen faces further adversity. After a purse-snatching incident leaves her injured, she experiences a cascade of misfortune and takes on two part-time jobs to make ends meet. Gerry continues his deceitful relationship with Lydia, who tries to undermine Helen by dropping hints about her affair with Gerry. As Helen suspects foul play, she is found to be pregnant.
A dramatic turn arises when Lydia invites Gerry over to break up; however, the truth comes crashing down as Helen arrives, leading to an explosive confrontation that leaves her mentally shattered. Both narratives culminate in Helen's hospitalization, where she tragically loses her baby. In the scenario where she met James, she dies in his arms, right after he vows to make her happy. Conversely, in the version where she missed the train, after recovering, Helen tells Gerry to leave her life for good.
As she departs the hospital, she drops an earring once again, and as fate would have it, James appears to return it, prompting a renewed connection. They share a warm exchange reminiscent of their first meeting, with Helen recalling the punchline of James's earlier joke, intertwining their fates once more.
