Dennis Doyle is an underachieving, overweight security guard with a penchant for making poor decisions, one of which was leaving his pregnant fiancée, Libby, at the altar five years earlier. Living a life filled with regret and self-loathing, Dennis is stuck in a rut of his own making, with his beloved Libby and their son being his only sources of true happiness. However, their relationship is strained, colored by his past actions and current lackadaisical approach to life. His world is upended when he discovers that Libby has started dating Whit, a successful and seemingly perfect American man who is everything that Dennis is not. Driven by a newfound sense of competition and a desperate desire to prove himself, Dennis is jolted out of complacency.

In a misguided attempt to win back Libby's affections and demonstrate a commitment to change, Dennis impulsively announces that he will run in the same marathon that Whit, an avid runner, has entered. Despite his physical unpreparedness and lack of training, Dennis embarks on a grueling journey to prepare for the 26.2-mile race, hoping to show everyone, especially Libby, that he can follow through on a commitment. Along the way, he is supported and challenged by his friends, who harbor doubts about his ability to finish the race. Dealing with his emotional insecurities and physical limitations, Dennis's endeavor to run the marathon becomes more than just a chase after Libby's heart—it transforms into a quest for personal redemption and self-improvement.