Sam finds himself in a cornfield in what he recognizes as November. He believes that there are pheasants nearby and mimics shooting at one as it flies up. Three cheerleaders are parked nearby and Sam recognizes one of them as Lisa, a girl from his high school. The other two cheerleaders are there to see if Sam wants to take Lisa to the upcoming dance. Sam runs off and finds himself on the road going to his childhood home in Elk Ridge, Indiana. He runs up to the door and sees his younger self in the reflection. His mother Thelma comes out and calls him by name.
As Thelma calls her husband John in, Sam stares at her. Surprised, she asks what the matter is and he hugs her for seemingly no reason. He hugs her again and admits he's glad to see her, and then goes to get John, who is milking cows in the born. John says that Sam must be late because he was practicing basketball, and warns that he won't be as good as his brother was when he was at All-State. Sam says that John looks just the way he remembers and hugs him, much to John's surprise. His younger sister Katie comes in and he hugs her as well, and she figures that he's trying to get Tom's room. He tells her she can have it.
That night, Sam is wolfing down his mother's food as everyone else stares. Sam stumbles when his father talks about basketball and almost tells them how the future game against Bentleyville will go. John warns that the key is stopping No Nose Pruitt. Al arrives as Katie explains that No Nose plans to kill Sam during the upcoming game because he's sweet on Lisa, and Sam asked her to the dance the last Friday. Sam hastily leaves to do his chores so he can talk privately. As he goes, Thelma admits she has a funny feeling about Sam.
Out in the barn, Al informs Sam that he's there to make sure that the Elk Ridge team wins against Bentleyville in the game just after Thanksgiving. The game was the turning point in a lot of people and Elk Ridge will go on to win the state championship. However, Sam tells his friend that he doesn't want to be out of there once he completes the Leap. John dies of a coronary in three years and Sam hopes to stop it. Katie elopes with Chuck, an abusive alcoholic. Most importantly, Sam's brother Tom comes home on leave for Thanksgiving before shipping off to Vietnam, where he's killed in action. Al warns that he can't change things that are meant to be unchanged, just like they couldn't stop Beth from marrying someone else. Sam insists that it's different this time and figures that this time, he is home. He thinks he's being rewarded and insists that he's going to do what he can to help his family.
The next morning, Sam hides his father's cigarettes and gets all of his father's chores done already. He gives him a cup of decaf coffee and insists that regular coffee isn't good for the heart. John sits down for Sam's healthy well-balanced breakfast. While John objects, Katie comes in with a protest T-shirt and Thelma tells her to remove it immediately. John insists that he's healthy because he works hard and eats dairy products, but Sam wants him to reverse the damage done by high cholesterol. He tries to explain the idea of aerobic exercise but John insists he's healthy and ignores Sam's advice. He goes into town to get cigarettes and Sam admits to his mother that he was trying to shock his father. Sam can't explain what he knows about his father's death in three years. Once Katie goes, Thelma gets Sam to admit he burned John's cigarettes. She agrees she'll try to cut out some fat, but doesn't want Sam to bring up John dying young.
Al arrives and Sam figures he's going to change the future. Al isn't convinced, and tells him he needs to get to basketball practice. Sam agrees to go but refuses to play in the game on Friday. Al points out that if Sam believes he's there to help his family, winning the game won't make a difference.
At the school gym, Sam is playing with the team while Lisa and the other cheerleaders practice. The coach calls them in and warns them about No Nose. He has someone even uglier to scrimmage against and calls in "Kong," a player wearing a gorilla mask. The team goes back to the court and Al appears. Sam is sure he knows Kong and goes back up against him. Kong steals the ball and Sam asks who he is. He finally realizes that it's his brother Tom.
Back at the farm, Sam and Tom go hunting and his brother describes his time in basic training. Sam admits he wants to stop Tom from going to Vietnam. He tries to explain that Vietnam is a losing battle and insists that Tom shouldn't waste his life on a lost cause. Tom admits that it's a possibility he could die but he doesn't plan to miss out on the action. He questions Sam's patriotism and insists he'll fulfill his duty, and is confident he won't be killed in Vietnam. When he wonders how Sam can be so sure what the future is, Sam tells him that he can see the future. He predicts that Tom is going to flush two birds, hit the first, and miss the second. A few seconds later, it comes true just as Sam described.
Later, Tom describes the incident to the family doctor and his parents. The doctor, Berger, notes that what Sam predicted is the same thing that happened dozens of times, and insists that Sam's a gifted and imaginative young man under pressure. When John wonders what kind of pressure, the doctor explains that Sam is going to college at 16 and being approached by some of the top technical colleges in the country. Berger figures Sam is subconsciously inventing a science fantasy to keep Tom from going to Vietnam. He tells the Becketts to play along with Sam, and later when the pressure is off, Sam will realize that he created the fantasy.
On the porch, Sam is playing the guitar for Katie and describing the future and various slang terms. He tries to steer her away from Chuck and explains that he has a drinking problem. Sam figures she's humoring him like the others. Katie asks if Paul McCartney is dead like the urban legends claim. Sam says that the Beatles split up but hedges around which of them dies. Instead he plays his favorite song, John Lennon's Imagine
. Katie breaks into tears as she realizes that Sam is telling the truth and playing a song that hasn't been written yet. It also means Sam is telling the truth about Tom dying. Al appears and insists that Sam isn't changing his family's future, and making their present miserable. As the rest of the family comes out, Sam angrily lies and admits that he made everything up, and runs off into the cornfield in tears.
As Al catches up to him, Sam wonders why they can't save the ones they love, and angrily yells to the heavens that he quits. He runs until his exhausted. Al tells him that in fact he's received a gift: the chance to talk to his father and sister one last time. A gift that Al would give anything to have.
The Becketts settle down for Thanksgiving dinner and Sam simply enjoys the time he has with his family. Afterward, Sam shoots hoops with Tom at the barn, and Tom tries to show him what to do against No Nose so he can impress Lisa. Sam remembers that Lisa marries No Nose and Al confirms they get married and divorced a few years later. Tom insists that Sam has to beat Bentleyville and get revenge for him, and shows him how to get his arm into his opponent's face. Sam offers him a deal: if he wins and gets Tom's revenge, he asks for one day. The day is April 8, 1970, the day that Tom will die. He asks Tom to find a hole and stay there for one day. Tom agrees to the wager.
On the day of the game, No Nose is all over Sam on the court and knocks him to the floor. Tom comes out and tells him to get his breath back. Sam reminds him to keep his promise and gets back up. As he talks to Al, John wonders why Sam is seemingly talking to himself. Sam takes the free throw and scores both times, bringing his team to within one point of Bentleyville.
No Nose continues to ride Sam and goes in to shoot. Sam knocks the ball away and his teammate recovers, and Sam heads down the court with 22 seconds to go. He pauses and looks up at his family, including his father. He says "bye" and then swings around No Nose and takes the shot just as time runs out. He scores and Elk Ridge wins. As the crowd goes wild, Al tells him that he's changed history. As Lisa kisses Sam, Al tells Sam that she doesn't marry No Nose or Sam. Finally he checks on Tom and confirms he's still killed in Vietnam. Sam tries to call out to Tom and leaps…into a firefight in Vietnam, at Tom's side.