As Dennis Doyle (Simon Pegg) stands at the brink of his wedding, anxiety grips him as he questions his readiness to marry his pregnant fiancée, Libby. On their wedding day, overwhelmed with doubt, he makes a rash decision to flee, leaving behind a mix of regret and unresolved feelings. Fast forward five years, and Dennis is stunned to learn that Libby has moved on with the suave but self-assured Whit, who has charmed her with his confidence and achievement. Driven by a fierce desire to win back her heart, Dennis resolves to take bold steps to prove his worth.
With the highly anticipated Nike River Marathon in London fast approaching, Dennis seizes this as a chance to not only challenge himself physically but also to revive Libby's interest in him. He recruits two eccentric "coaches" to assist him: Gordon, Libby's cousin and his loyal friend who has placed a significant bet on Dennis's ability to succeed, and Mr. Ghoshdastidar, their quirky landlord who adopts unconventional training techniques that even involve a spatula.
As the marathon day nears, Whit takes the opportunity to propose to Libby during her birthday party, thrusting Dennis into a deeper despair. Just as he considers dropping out of the marathon altogether, a heartfelt moment with their young son, Jake, transforms his outlook. After observing Jake facing his own struggles with rejection from a girl due to another boy’s ponytail, Dennis realizes it’s time for him to face his fears head-on.
Emboldened by this new clarity, he recommits to the race, igniting a fierce competition between himself and Whit. The tension escalates as they push past professional runners, culminating in a dramatic moment where Whit maliciously attempts to trip Dennis, resulting in a serious injury.
As medical help arrives, Libby rushes to assist Dennis, only to find Whit pretending to be a victim, accusing Dennis of wrongdoing. A doctor’s revelation that Whit has merely "hit the wall" serves as a sobering reminder of the repercussions of such deceit. This upsetting incident disturbs young Jake, who hears Whit call him a "little shit," shattering his innocence.
Despite his injuries, Dennis refuses to give up, battling through pain and fatigue with the support of his friends, Gordon and Mr. Ghoshdastidar, rallying around him. He perseveres, driven by a sense of purpose beyond the physical pain he endures. As exhaustion begins to take its toll on him, a pivotal moment of clarity ignites his determination to cross the finish line.
Meanwhile, at Whit's house, Libby, Jake, and guests anxiously watch footage of the race that unveils the truth about Dennis’s struggle with Whit. As it becomes clear that Whit's earlier claims were nothing but lies, Libby reaches a breaking point. With a resolute declaration of "nobody's perfect," she places her engagement ring on the table and leaves with Jake to find Dennis at the finish line.
In a touching display of strength, just as Dennis stumbles close to the finish, the encouragement of Libby and Jake reinvigorates him. With their support, he finds the energy to sprint towards the end, collapsing into their arms as they celebrate his hard-fought victory.
In the final moments, Dennis pays a visit to Libby's home to invite Jake for an outing and hints at something important to discuss later with Libby, who agrees to go on a date with him. The film wraps up with a humorous scene of Gordon as he entertains two elegantly dressed women with stories of his escapades aboard his luxurious yacht, showcasing his unique style—or lack thereof— in a lighthearted manner.
