When Gym Candy begins, Mick Johnson recalls the first time his father, Mike, took him out to play football. Mick was four years old and took to it instantly but was also driven to succeed because of his father’s legacy. Mike Johnson was drafted into the NFL by the San Diego Chargers, and as Mick progresses through Pop Warner football and up through the junior high leagues, his desire to be as good as his father is a primary driver. When Mick learns that his father’s short-lived career was not the result of an ankle injury, as he had been told, but rather a series of incidents involving the law, drugs and alcohol, poor play, and drunken fights in clubs, he has to rethink why he wants to succeed at football so badly.

By the time he is in eighth grade, Mick is talented enough that the high school coach invites him to spring training camp to try and earn a spot on the varsity roster for the fall season. Mick and two of his friends—Drew and DeShawn—make the team and then spend the summer practicing together. As the season begins, however, it becomes obvious that Coach Downs does not start freshmen. No matter how talented he may be as a running back, Mick is relegated to the special teams unit. His father pressures Mick to do better, in order to force Downs to play him, adding to Mick’s unhappiness.

Early in the season, the starting quarterback and running back are caught drinking. Coach Downs suspends them, and Mick and Drew are given the chance to start. During a pivotal game against Foothill, Mick has the chance to score the winning touchdown, but he is stopped by a stronger player one foot from the end zone. After the loss, Coach Downs suggests that Mick needs to start lifting weights. Mick commits to gaining strength and lifts consistently in the high school weight room, but his father tells him that his coach’s methods are outdated, and the weight room is dilapidated. He gets Mick a membership at Popeye’s and sets him up with a trainer named Peter Volz. Peter introduces Mick to oral steroids, promising him that they will make him unstoppable on the field. Mick says no, but later, when another player is threatening to take his spot, Mick relents and begins buying steroids from Peter.

The results come quickly, but so do the side effects. Mick is faced with acne, breast tissue, sudden depression, and fits of rage that accompany his new stamina, strength, size, and prowess on the football field. While at a lake with friends, Mick is embarrassed to take his shirt off, but he does, and they notice his acne and developing breasts. However, no one mentions it. When he is at Popeye’s, he asks Peter why he doesn’t have the same symptoms even though he uses steroids. Peter says this is because he uses a steroid “stack,” which means that he injects himself. Mick decides to try an injectable drug called XTR. The physical results are better than the previous steroids, but the mood swings are just as bad.

During a pivotal game, Mick is overcome with anger after a play ends. He tackles another player after the whistle and is ejected from the game. Coach Carlson suspends him for one game. When Mick returns to action two weeks later, he helps the team beat Foothill, their nemesis from his first season. However, Drew had been watching Mick disappear into the locker room stall with his duffel bag and started to suspect that Mick was hiding something. When Mick gets home after the game, his vial of XTR is gone. Drew calls him and says they have to talk. He tells Mick that he is going to tell Carlson about the steroids, and he says that none of the wins count for anything because Mick cheated.

Mick takes out a pistol that he has brought and attempts to commit suicide. The bullet misses his brain due to a misfire and Mick survives. As the novel ends, he is in a twenty-day drug rehab center, unsure of whether he is grateful to be alive. Mick knows that he will be allowed to return to football in the future if he chooses, but he also knows that the temptation to use steroids will always be there.