Sam finds himself in a cornfield in what he realizes is the November harvesting season. He leaves the field and sees three cheerleaders parked nearby... and Sam recognizes one of them as Lisa, a girl he went to high school with. Lisa's friends want to see if Sam is going to take Lisa to the homecoming dance. Sam hastily leaves and recognizes the road he's on as leading to his home in Elk Ridge, Indiana, when he was boy. When he gets to the house, he sees his reflection and realizes that he's in the body of his own younger self, as his mother Thelma comes out and calls him by his own name.
As Thelma calls her husband John in, Sam enthusiastically hugs her, much to her surprise. He goes to get John, who is in the barn milking cows. They discuss Sam's high school basketball and John figures that he may not be as good as his older brother Tom was at All-State. Glad to see his dead father, Sam hugs him as well as his sister Katie when she comes in to see what's going on. Both of Sam's relatives are surprised and Katie wonders if he's trying to get Tom's room. A relieved Sam tells her that she can have it.
That night, the Beckett family is having food and Sam is eating the home-cooking. John talks about the upcoming Elk Ridge game against Bentleyville and Sam almost gives away what he knows of how the game will come out. As they discuss Bentleyville's leading player, No Nose Pruitt, and how he hates Sam because he's sweet on Lisa, Al arrives and tells Sam that they need to talk privately. Sam says that he needs to do the chores and goes outside while Thelma wonders why she has a funny feeling about her son.
As Sam works on his chores, Al tells him that he's there to make sure that Elk Ridge wins against Bentleyville in the annual game. If Sam helps his team win the game, they go on to win the state championship and affect the lives of dozens of people. However, Sam doesn't want to leave. He reminds Al that John dies of a coronary, Katie elopes with an abusive alcoholic, and Tom is killed in Vietnam. Sam wants to stay and prevent all of those things from happening. Al warns him that they can't change their own pasts but Sam believes that he Leapt there as a reward so he could help his family, and plans to take advantage of the situation.
The next morning, Sam starts by burning John's cigarettes, doing his chores, and giving him decaf coffee, all to keep him from dying of a heart attack. John refuses to go along, insisting that he lives a healthy life because he works hard for exercise and eats a lot of dairy products. Sam tries to convince him to avoid cholesterol and do aerobic exercise, warning that he could die if he doesn't. While they talk, Katie comes in wearing a anti-war t-shirt and Thelma tells her to take it off. Disgusted, John goes to town to get more cigarettes, and Sam admits to his mother that he destroyed them. Thelma agrees to cut down on the fat in their diet, but tells him to stop talking about John dying. Al comes in at the tail end of the conversation and realizes what he's doing, and tells Sam to focus on winning the game. Sam wants to lose so that he doesn't Leap and can help his family, and Al points out that if he truly believes he was Leaped to save his family, then what he does won't matter.
At the high school gym, Sam is practicing with the team but both he and Al are distracted by Lisa and the other cheerleaders practicing their routine. The coach warns them about No Nose and brings in an unknown player wearing a gorilla mask. He has the mystery player scrimmage against Sam, who thinks he recognizes the man. When the player steals the ball from him, Sam realizes that it's his brother Tom.
The brothers go back to the Beckett farm and hunt, and Tom talks about basic training and how he's home for Thanksgiving before shipping out to Vietnam. Sam tries to stop him without explaining his foreknowledge, saying that Vietnam is a lost cause and Tom shouldn't throw his life away on it. Tom doesn't believe that he'll die and feels he should do his patriotic duty, and wonders what's gotten into Sam. Sam explains that he can see the future and accurately describes what will happen in a few seconds when Tom flushes two pheasant.
Later at the farm, John and Thelma call in the family doctor, Berger. He dismisses Sam's prediction as a coincidence and his concern as a way of relieving the stress of going to college at 16. Berger explains that Sam is subconsciously inventing a fantasy to cope with the pressure and his concerns about Tom, and tells the Becketts to play along until Sam grows out of it.
On the porch, Sam is with Katie and playing his guitar. He talks about the future he knows and mentions that Katie will be dating Chuck several years in the future and eventually marries him. Humoring him per the doctor's instructions, Katie asks about the Beatles and whether Paul is dead per the rumors. Sam can't tell her that John Lennon will be shot, but starts playing "Imagine." Realizing it's a song that hasn't been written yet and that Sam is telling the truth about Tom, Katie breaks into tears. As the rest of the family comes out to investigate the commotion, Al returns and warns Sam that he's not changing anything: he's just making his family miserable. Sam angrily tells his parents that he made everything up and then runs off, crying. In the field, he wonders to Al why they can't save the ones they love, and angrily yells at God that he's done. His friend tells him that even if he can't do anything, Sam has the chance to be with his family one last time... something that Al would give anything for.
Sam accepts his friend's advice and simply enjoys being with them for Thanksgiving. After dinner, Sam and Tom shoot hoops and Tom tries to coach him on how to beat No Nose. When they talk about Sam impressing Lisa, Sam remembers that she marries No Nose anyway. Al chimes in to confirm that the couple get divorced a few years later. When Tom shows Sam how to deal with No Nose and win the game to avenge his brother, who lost against Bentleyville, Sam offers his brother a deal. If he wins, he wants one day from Tom: April 8, 1970, the day that Tom dies. He asks Tom to avoid combat on that one day, and Tom agrees.
On the day of the game, No Nose rides Sam throughout the game, knocking him to the floor. When Tom comes out to help him, Sam tells him to remember his promise. As he goes out to take the free throw, Sam talks to Al and John wonders why his son is apparently talking to himself.
Sam makes both points on the free throw, bringing Elk Ride within one point of Bentleyville. With time running out, Sam steals the ball from No Nose and heads down court. He looks up at his cheering family and whispers "goodbye," and then takes the shot. He scores and Elk Ridge wins. As the team hoists Sam up on their shoulders, Al tells his friend that he changed history. Lisa doesn't marry No Nose... or Sam. However, when he checks Ziggy, Al has no choice but to tell Sam that Tom still dies in Vietnam. Sam reaches out to brother to try to warn him... and Leaps. He appears in Vietnam in the middle of a firefight--standing next to Tom.