The protagonist, Alfred Brooks is hanging out with his best friend, James, and James's friends Major and Hollis. The three of them want to break into the Epsteins' grocery store where Alfred works. Although Alfred does not go with them, he forgets about the new alarm system that the Epsteins just installed. James gets caught by the police, and later that night, Major and Hollis beat Alfred up. A man from the neighborhood, Henry, takes Alfred home. Henry mentions that he has been working for Mr. Donatelli, who owns a gym where he trains some famous boxers. Alfred goes to check out the gym and tells Mr. Donatelli that he wants to be special—he wants to be a champion. Donatelli tells him that first, he has to be a contender—someone for whom it is in the realm of possibility to be a champion.
The next night, Henry, Spoon, Jelly Belly, and Mr. Donatelli take Alfred to a boxing match at Madison Square Garden. Mr. Donatelli's most promising boxer, Willie Streeter, is competing in an important match. During the match, Streeter gets hurt, and Donatelli eventually concedes. Although Streeter is angry, Spoon says that Donatelli is the best manager around.
Alfred begins training for boxing. He runs, eats, and sleeps exactly in the way Mr. Donatelli prescribes. After six weeks of very hard work, Alfred becomes discouraged because he has never been in a real fight—he has only trained, exercised, and hit punching bags. Succumbing to his frustration and to his loneliness in having been separated from James, Alfred goes to a party thrown by Major, partially because of a promise that James would be there. Alfred resists the drugs and alcohol at first, but then he indulges. Eventually, James shows up but Alfred barely gets to speak to him—James is more interested in the heroin that Major offers him.
Alfred goes to Coney Island with Major the next day, narrowly missing a run-in with the police for Major's possession of a stolen car. The next few days Alfred is in a stupor and eventually goes back to the gym to get his clothes and equipment. While there, he asks Mr. Donatelli if he could have been a contender. Mr. Donatelli answers that he does not know and that he never knows the answer to that question until he sees how a boxer reacts to getting hurt. Needing to know the answer to that question himself, Alfred begins training again.
Alfred's progress is speedy. He learns that Mr. Epstein used to box as well, and the two of them begin to develop a renewed trust at work. Despite the protests of his Aunt Pearl, he trains until he is ready for his first fight. Spoon, Henry, and Jelly Belly take care of him that day by letting him relax, eat, and sleep. Alfred wins his first match by majority decision. Aunt Pearl questions him later, and he admits that he did not enjoy winning much but that he refuses to quit.
During his second fight, Alfred hits his opponent so hard that his opponent gets knocked unconscious. Alfred is sickened by the scene and is upset at the brutality of his own actions. Mr. Epstein, proud and impressed, pays for Alfred's lessons. Spoon tells Alfred that he should consider going to night school, and Alfred later reveals that he is planning on getting his high school diploma.
After his third fight, Donatelli tells Alfred it is time to retire. He says that Alfred does not have the requisite killer instinct. Alfred protests and says that while that may be true, he still doesn't "know," yet, because he has never been injured in the ring. Donatelli eventually lets Alfred persuade him into fighting one more match. Alfred faces an opponent who is bigger, stronger, and better in his final match, but he lasts through all the rounds. Although he loses by majority decision, he knows that the strength and bravery he displayed in the round is something of which to be proud. Satisfied, he retires.
That night Alfred learns that James has tried to break into the Epsteins' store again. Aunt Pearl tells him that the police are looking for James and will probably find him, because James cut himself and was bleeding badly. Alfred finds James in their old hideout in Central Park and convinces James to go to the hospital. He assures James that together, they will beat his drug addiction, find him a job, and help him get back on track.