A previously undefeated fairground boxer named “One Round” Jack Sander (Carl Brisson) is beaten in the ring by a mysterious challenger, who is later revealed to be professional boxer and World Heavyweight champion Bob Corby (Ian Hunter). Bob’s manager is impressed with Jack’s performance and offers Jack the chance to become Bob’s full-time sparring partner, on the condition that he win a trial fight to be arranged at a later date.
Bob begins spending more time with Jack’s girlfriend Mabel (Lillian Hall-Davis) and buys a bracelet for her to express his feelings. The two kiss but Mabel reluctantly puts a stop to it. The next day when Jack inquires about the bracelet Mabel lies to Jack, telling him that Bob bought it for her because he didn’t want to take the money.
Jack wins his trial fight and is made Bob’s official sparring partner. Keeping his earlier promise to Mabel he agrees to marry her the next day. Mabel goes through with the wedding although somewhat reluctantly due to her new found feelings for Bob. At the wedding reception Bob jokingly states that he wishes Mabel had been the prize at his and Jack’s original fight. Jack boldly states that he would defend his wife in a fight against any man. A friendly exhibition match is arranged between the two fighters which Bob wins. After the fight Jack sees that his bride flirting with Bob and suspects that they are having an affair. Jack declares his intent to fight Bob for the heavyweight championship of the world but he is told that this can not happen yet as Jack is not ranked high enough in the league to challenge Bob to an official fight. Jack instead works his way up the rankings and eventually becomes the number one contender.
Jack arranges a party with his friends in his apartment as a way to surprise Mabel and let her know that he will be fighting for the world title. Jack and his friends wait long into the night but Mabel does not show up. After Jack’s friends leave, Jack stays up and waits for Mabel and eventually she sees her getting out of Bob’s car. Jack angrily confronts Mabel about her liaisons with Bob and smashes a framed picture of him. Jack then goes to the club where Bob is and confronts him, Bob tries a cheap shot but Jack knocks him out before he connects. Jack informs Bob that he is officially the number one contender and they will settle their differences in the ring.
On the day of the fight the two fighters seem evenly matched until the final rounds where Bob starts to dominate Jack. Jack considers giving up until Mabel, seeing him in pain, runs over to Jack’s corner and declares that she wants to be with him, not Bob. Jack musters up his remaining energy and unleashes a flurry of punches in the final round, eventually knocking Bob out and winning the fight. Jack and Mabel embrace as Bob accepts defeat.