Helen, a young PR executive in London, is fired from her job and rushes to catch an early train home. From this pivotal moment, the film diverges into two parallel storylines: in one timeline, Helen catches the train and in the other, she misses it. In the version where she catches the train, she meets James, an affable stranger who sparks her curiosity and provides comfort during a difficult time. She arrives home earlier than expected and makes a startling discovery about her boyfriend, Gerry, leading her to reevaluate their relationship.

In the alternate timeline, where Helen misses the train, she faces a series of delays that keep her unaware of Gerry's deceit. Consequently, she continues to navigate life under a heavy cloud of professional and personal uncertainty, searching for new job opportunities and increasingly trying to support Gerry's attempts at being a successful novelist. As both narratives unfold, Helen’s life takes on drastically different trajectories, illustrating how seemingly minor events can set the course for our fate, influencing the people we meet and the choices we make.