Eighteen-year-old David Watson returns to Rossmore University with his friend, Barrett, after secretly painting a mural of a forest in a nearby vacant lot throughout the night. Arriving at the Lambda Chi Delta fraternity house, Russ, the fraternity president, and his two friends, J. P. and Mikey, confront David about the term papers they ordered David to write for them. During his American History class, David stares at Robin, the girl he has crush on. Barrett reminds him that Robin is Russ’s girl friend. In the evening, David and his parents, Arnold and Betty Watson, arrive at the mansion of his grandfather, Jack Watson, for Jack’s eighty-first birthday celebration. During the party, Jack’s companion, Madelyn, flirts with David. After David gets away from her and joins his grandfather, Jack brags to his friends that his grandson is just like him. However, David says he could never be confident and suave like Jack. As his birthday cake is brought out, Jack wishes he were eighteen years old again. With David’s assistance, Jack and David blow out the candles. After the party, David and Jack go to a diner to spend some time together. Driving home, Jack loses control of the car. As Jack clasps David’s hand, the vehicle crashes into a storefront window and both men are knocked unconscious. At the hospital, Jack awakens, but discovers his soul is somehow inside David’s body. Seeing his own body in a coma, Jack realizes David’s soul is trapped. Excited to relive his youth, Jack goes back to campus. Attending art class, Jack comments on art being a waste of time. Surprised, Barrett tells Jack this is his favorite class. During History class, Robin tells Professor Swivet she is unable to find information on former president Harry S. Truman’s middle name. Proclaiming he is an expert on Truman, Swivet states it is only an initial. Jack interrupts and says it stands for “Sergei,” the name of Truman’s uncle from Eastern Europe. When Swivet questions Jack’s source, Jack says Harry Truman himself told him. However, Jack remembers he in David’s body, and reiterates that his grandfather told him. Later, Jack and Russ compete in a race at the track meet. Seeing Robin watching him, Jack finishes second behind Russ, and qualifies for the following week’s Midwest Regional Conference. Afterward, Jack approaches Robin and offers to tell her about Truman if she will go to dinner with him. Robin accepts. In the middle of the night, Barrett and Jack go to finish the mural in the vacant lot. His grandson’s artistic talent impresses Jack. Later, Jack goes to the hospital and sees Charlie, his best friend. Trying to convince Charlie that his soul is inside David’s body, Jack recalls the time he and Charlie were snowed in for days with two women. Knowing Jack would never tell his grandson such a story, Charlie believes him. Jack goes to tell Madelyn as well, but believing Jack to be David, she begins flirting with him. Realizing Madelyn has been using him for his money, Jack leaves. Returning to the fraternity house, Jack sees Russ and the others playing poker. Joining them, Jack uses his card-playing skills and wins everyone’s money. For the final hand, Jack bets Russ $1,000 that he will win at the Midwest Regional Conference. Later, Jack meets Robin for dinner. Having a great time, Robin asks Jack out on another date. The following day, Jack goes to his company, Watson Dials, and finds Arnold in his office. Seeing his son efficiently running the family business, Jack regrets taking Arnold and his ideas for granted in the past. Robin takes Jack to a punk rock club, but Russ arrives and sees them. After Russ punches Jack, Robin tells him to leave her and Jack alone. Later, Jack’s fraternity throws a party. Finding the term papers David had been coerced into writing by Russ, J.P. and Mikey, Jack throws them into a bonfire. Russ attacks, but Jack fights back. Afterward, Charlie arrives and tells Jack that Arnold and Betty have taken David off life support. Driving to the hospital, Jack confesses to Charlie that he had recently updated his will and left one half of his business to Madelyn. Finding David on a gurney, Jack is relieved that he is still alive. Jack pushes the gurney through the hallways, yelling at David to wake up. As David’s hand touches Jack’s hand, they crash through the stained glass window into the hospital chapel. The impact switches their souls back into their own bodies. Jack tells Arnold he is sorry for not being the best father, and not to make the same mistakes with David. Afterward, Jack breaks up with Madelyn and tells his butler, Horton, to call his lawyer so he can remove Madelyn from his will. The next day, David prepares for the Midwest Regional Conference. Jack arrives with his old track shoes for David to wear for his race. When David asks how he even qualified, Jack promises to tell him later. During the race, David struggles to keep up with Russ. However, the crowd cheers his name and Robin blows him a kiss. Sprinting forward, David passes Russ and crosses the finish line first. Afterward, Russ grudgingly hands Jack $1,000 from their bet. While David looks confused, Robin kisses him and says she will see him later to talk more about Harry S. Truman. Afterward, David asks his grandfather to tell him everything he knows about Truman. Jack happily agrees.