In 1869, two boys bury a chest near Brantford, New Hampshire. When one boy asks what will happen if someone unearths it, the other replies "May God have mercy on his soul." The sound of tribal drums is heard as the boys ride away.
A century later, in 1969, 12-year-old Alan Parrish (Hann-Byrd) flees on his bicycle from a gang of bullies, then runs into the shoe factory owned by his father, Sam (Hyde), where he meets his friend Carl Bentley (Grier), one of Sam's employees. When Alan accidentally damages a factory machine by clogging it up with a shoe that Carl designed, Carl takes the blame and loses his job. Outside the factory, after the bullies beat him up and steal his bike, Alan follows the sound of tribal drumbeats into a construction site and finds the buried chest, which contains a board game called "Jumanji".
Alan takes the game home and has an argument with Sam, who insists on sending him to boarding school. Alan prepares to run away from home, but his friend Sarah Whittle, who is the lead bully's girlfriend, arrives with Alan's bicycle. Alan and Sarah begin a game of Jumanji, which behaves strangely: When a player rolls the dice, the player's piece moves itself and a message appears on the board. When Alan makes his first move, a vortex sucks him into the board, trapping him in an unseen jungle. He will be freed when a player rolls a five or an eight, but Sarah abandons the game after being attacked by African bats.
Twenty-six years later, in 1995, Judy and Peter Shepherd (Dunst and Pierce) move into the Parrish house with their aunt Nora (Neuwirth) after the deaths of their parents in a car accident. They hear Jumanji's drumbeats and find the board game in the attic. When they begin playing, they are attacked by giant mosquitoes and crazed monkeys. The instructions say things will return to normal once the game is done, so they continue. Peter rolls a five, releasing both a lion and Alan (Williams), who is now an adult. Alan locks the lion in a bedroom, then goes to the shoe factory, which is now closed down. On the way, he meets Carl, who works unhappily as a police officer, and discovers that the town's economy has been devastated by the factory's closure. In the factory, a stranger tells Alan that Sam abandoned the business to search for him until his death.
When rolling the dice has no effect on the board, Alan realizes they are continuing the game he and Sarah started in 1969. The next move is Sarah's. They find Sarah (Hunt) at home, a reclusive outcast traumatized by the game and its aftermath. Alan tricks her into rejoining the game, and the following moves release man-eating vines, a hunter named Van Pelt (Hyde), a stampede of rhinos, elephants and zebras, and a pelican that steals the board. Increasingly relentless havoc ensues throughout the town. Among other things, Peter starts turning into a monkey after trying to cheat; Peter, Sarah and Judy battle Van Pelt in a local discount store; a monsoon floods the house; and an earthquake splits it in two. Finally, Alan wins the game just as Van Pelt is about to kill him. Van Pelt and the other jungle elements are sucked back into the board.
With the game over, Alan and Sarah find themselves as children in 1969 again, but they both remember the game. Alan admits his guilt for destroying Carl's shoe, Carl gets his job back, and Sam tells Alan he doesn't have to go to boarding school. Alan and Sarah then throw the Jumanji board into a river.
Twenty-six years later, Alan's and Sarah's knowledge of their experiences during the game has changed the future for the better: Alan and Sarah are married and expecting a child, Alan's parents are still alive, Alan has taken over the shoe business, and Carl still works there. When Judy, Peter, and their parents visit the Parrishes at a Christmas party, Alan and Sarah offer Judy and Peter's father a job in the shoe company.
Sometime later, two French-speaking girls hear drumbeats as they walk along a beach, where the Jumanji board is half buried in the sand.