In a fairground boxing ring, 'One Round' Jack Sander challenges all comers to hold out one round against him. But he meets his match in Bob Corby, and he is humiliated by his defeat until he learns that Bob is in fact a boxing champion. Bob's promoter, James Ware, is impressed by Jack, and offers him the chance to join his stable. Meanwhile, Bob is taking an interest in Jack's fiancée Mabel, and while Jack is negotiating with Ware, Bob gives Mabel an arm bracelet as a gift.

When Jack wins his trial fight, he and Mabel marry, but Mabel is already torn between Jack and Bob. Jack's career begins to take off, but Bob and Mabel are spending more and more time together, and Jack fears he will have to fight Bob to get her back.

When Jack wins his latest bout, he invites his old friends from the fairground home to celebrate, but the evening is a disaster and the others leave when Mabel fails to appear, being out on the town with Bob. When Mabel returns, she and Jack argue, and Jack goes off to confront Bob at a nightclub. Jack floors Bob with one punch, challenging him to a rematch in the ring, then storms home, only to discover that Mabel has left.

On the night of the big fight, Jack's friends see Mabel on her way to Bob's dressing room, but resolve not to tell Jack for fear of upsetting him. However, Jack spots Mabel in the middle of the fight, and his distraction gives Bob the chance to deliver a knockout blow. The bell saves Jack, but after another round defeat is looking certain until Mabel, moved by Jack's suffering, switches her allegiance and rushes to his corner to offer her support. Bolstered, Jack finds his second wind and launches himself on Bob, finally landing a knockout punch. Mabel joins Jack and his team in celebrating. After the fight, one of Bob's team shows him the arm bracelet, found in the arena where it was discarded by Mabel.