In Chicago, Illinois, amateur boxer Leon Johnson wins two division titles in the Golden Gloves competition. At the awards ceremony, reporter Julie Winters hands out trophies, but when she calls Leon’s name, he is in the men’s room making love to a fan and his manager, Charles “Charlie” Golphin, has to get him. Leon is excited to meet Julie and asks her to dance. She wonders if he plans to become a professional boxer, but he says he is a lover, not a fighter. Later that night, Leon learns that his little sister, Kelly, had a medical crisis and is in the hospital. Kelly has sickle cell anemia and Leon is frustrated that she is not getting the best care possible because they are not rich. He decides to become a professional boxer to finance Kelly’s recovery. Mrs. Johnson wants her son to remain in college to become a doctor, but he refuses. Leon asks Charlie to manage his professional career and offers a percentage of his earnings. Charlie wants to talk to a local trainer, but Leon insists they learn from the best. The next day, they arrive at Muhammad Ali’s gym. Ali is not interested at first, but Leon says he can beat anyone and gets into the ring to prove it. Ali is impressed and assigns Frankie as Leon’s new trainer. Frankie is a rigorous trainer, and Leon continually wins, but realizes that he needs a gimmick to get more people to attend his fights. Julie is assigned to report on “Leon, The Lover,” and at Leon’s next match, she sits next to Kelly and a nurse. Leon hands out roses and kisses to the women as he enters the arena. During the fight, Leon flirts with Julie before knocking out his opponent. Afterwards, Leon makes front page news; he kisses Julie and announces that he wants to take on the defending world welterweight champion, “The St. Louis Assassin.” Leon and Julie have dinner and she admits that she is impressed with how he treats his sister. He says he will be worthy of Julie’s admiration once he earns more money and titles, but she insists those things do not matter to her. Their romance progresses as Leon trains hard and wins fights, and eventually she accepts his engagement ring. Fight promoter Big Man and his henchman, Tony, discuss giving Leon a chance at the championship, but they want to own him first. Big Man orders Tony to exploit Leon and Charlie’s weaknesses. Tony invites them to a party where Charlie is introduced to cocaine, and Leon is seduced by a prostitute who wants him to prove his reputation as “Leon, the Lover.” Later, Leon’s fight against The St. Louis Assassin draws an overflow crowd, and Leon wins. Julie wheels Kelly into the locker room as Leon announces his plan to fight world champion Iceman next. Big Man represents Iceman, and meets with Leon and Charlie to offer his backing for the fight. If Leon wins, Big Man promises to make him an international success. In exchange, Big Man will own fifty percent of Leon’s take. Charlie is adamantly against the deal, but Leon worries that Charlie’s escalating drug use is clouding his judgment. The argument ends their friendship, and Leon signs with Big Man. The fight against Iceman is brutal and continues for thirteen rounds of vicious blows. Covered in blood, Iceman’s eyes are almost swollen shut when an exhausted Leon rallies and attacks. Iceman will not give up, but Leon is declared the winner. Later, Dr. Bachman informs Leon that no matter how much money is spent, there is no cure for Kelly, and Dr. Bachman agrees to let her come home from the hospital. At a party that night, Tony offers to help take Leon’s mind off his troubles and hands him the key to a bedroom where three prostitutes wait for him. Julie is also at the party and Big Man sends her to the room, saying that Leon is waiting for her. When Julie finds Leon in bed with the prostitutes, she ends their relationship. He tries to convince her that he loves her, but she realizes he has been changed by the attention and money. Months later, Leon is enjoying his wealth, fame and women when he gets a frantic call from Julie. Charlie is on a drug binge and acting crazy; Leon arrives as Charlie, in a straight-jacket, is taken away in an ambulance. It has been almost a year since Leon fought Iceman, and Big Man needs a new champion in order to pay off his large debts. He orders Leon to lose the upcoming fight with Mad Man Santiago, a vicious, rule-breaking fighter who is Big Man’s latest discovery. Big Man gives Leon $200,000 to bet against himself, claiming Leon can make $1 million and retire. Leon visits Julie in the middle of the night and tells her that Big Man is betting against him. He drops the envelope with $200,000 on her table and says he is going to do it so that Kelly is taken care of. Leon admits he loves Julie and needs her. The next morning Leon wakes up alone, and notices the envelope is gone. Julie returns moments later and announces that she returned the money to Big Man. She could never love a man who would throw a fight, but she loves Leon. As Julie borrows against everything she owns to bet on Leon, he regains his incentive to win. Leon reassembles his original team and trains with the help of Muhammad Ali, Frankie and Charlie, who is now clean and sober. At the fight, the odds are against Leon, and Big Man again offers him $200,000 to throw the fight, but Leon turns it down. Muhammad Ali, Julie, Kelly and Mrs. Johnson root for Leon during the brutal match. Leon is knocked down twice during the first round, but keeps fighting. Santiago fights dirty and, at one point, even the announcer claims he should be disqualified, but the referee ignores Santiago’s illegal moves. Round after round, Leon barely manages to stay on his feet, but when Santiago makes a joke about Kelly, Leon rallies. As they enter the final round, Leon is so far behind that it will take a knock-out for him to win. Santiago only has to steer clear of Leon, but he is intent on beating Leon and attacks. Leon knocks Santiago out cold and wins. Big Man is surrounded by debt collectors, while Leon is swarmed by cheering family, friends and fans.