In 1869, a pair of boys named Caleb and Benjamin venture into the woods near Brantford, New Hampshire to bury a chest emitting strange sounds similar to tribal drumbeats. They’re so desperate to get rid of it that not even the howl of a wolf can faze them. As they cover the chest with dirt, Benjamin wonders what would happen if someone finds it. All Caleb can cryptically say is: “May God have mercy on his soul.”
A century later in 1969, Brantford is bustling. As young Alan Parrish - whose father Sam owns the town’s shoe factory - is riding home, he gets chased by a gang of bullies, led by Billy Jessup, who is mad at Parrish for apparently hanging out with his girlfriend Sarah Whittle. The bullies chase him into the shoe factory. Once inside, Alan is greeted by employee Carl “Soleman” Bentley, who shows him a prototype for a slick, stylish sneaker (which totally isn’t Product Placement for a Nike Barkley 94) that he believes will revolutionize the industry. Sam sees Alan hiding from Billy Jessup, telling him that he needs to stand and face his fears like a man, leaving Alan disappointed in his dad's faith in him. In the process, Alan absentmindedly places Carl’s sneaker on a conveyor belt, damaging the machine and the shoe Carl hoped to present. Carl reluctantly takes the fall for Alan.
As Alan walks back outside, Jessup and his waiting cronies beat him up and steal his bike. However, Alan’s mind drifts elsewhere, as he feels drawn to a nearby construction site, hearing drumbeat-like sounds, leading him to a wall of mud and roots. Digging through, he finds the chest buried there, chops off its lock and opens it, lifting up a wooden game board with pictures of African animals and foliage. Spread front and center is the word "Jumanji".
Alan takes the game home to his family's mansion and opens it up, taking out four animal-shaped game pieces. When Alan's mother Carol startles him, he drops the pieces onto the board, where two of them magically stick themselves at the "Start" spaces without his knowledge. Later that night, Alan is eating dinner while Sam and Carol prepare to leave to attend a party where Sam will make a speech as guest of honor. Before they do so, Sam tells Alan that because of what happened earlier, they have decided that he is responsible enough to attend the Cliffside School for Boys. Alan, however, isn't happy about this and an argument erupts between him and Sam. Angry, Alan decides to try and run away from home.
As he packs his things (including Jumanji) and prepares to head out the door, Sarah Whittle comes over with Alan's bike. His leave is interrupted by the sounds of drumbeats that Sarah hears too. At first, Sarah doesn't want to play, but she accidentally rolls the dice, causing one of the game pieces to start moving on its own. A cryptic riddle appears in the game's center - "At night they fly, you better run, these winged things are not much fun." - and the two kids hear shrieks in the fireplace. Alan tries to put the game away, but he's startled by a clock and he accidentally rolls his turn. Another riddle appears - "In the Jungle you must wait, until the dice read five or eight". To his and Sarah's horror, Alan suddenly turns into a whirl of sand and is sucked into the game, while a large cloud of bats swarm out of the fireplace and chase Sarah out of the Parrish mansion before she can roll.
In 1995, the long-vacant Parrish home is moved into by three new people, Nora Shepherd - who plans to turn the place into a B&B - and her niece and nephew, Judy and Peter, who lost their birth parents in a car crash during a skiing trip. As a result, Judy tends to exaggerate or lie about things while Peter is almost completely silent around people he doesn't know. Aside from a few things - including Peter finding a straggling bat and the kids learning of the rumors about the disappearance of Alan Parrish - they adjust, but not without grieving.
A few days in, the two kids are preparing to leave for school when they hear drumbeats coming from the attic. Under a stack of board games is Jumanji, waiting for them to play it. Unable to move the two pieces already in spaces, they proceed to roll the dice. Their first roll summons a swarm of mosquitoes the size of baseballs, who escape out of a window after Judy knocks one into it. Their second roll isn't any better - it summons a bunch of chaotic monkeys in the kitchen. Judy finds a notice in the game board - anything that happens because of the game will go away when one of the players win.
Peter rolls again (he got doubles) and gets a 5. This is both good and bad; it summons an unnaturally buff and scary lion, but out of the game comes a now-adult (and definitely ungroomed) Alan Parrish, who traps the lion in bedroom. Realizing he's home, Alan joyfully gives many thanks to Judy and Peter and calls out for his parents...only for it to slowly dawn on him that he's been gone from Brantford for a long time. He rushes outside and jumps onto a police car being driven by Carl Bentley, who became a cop after getting fired from the shoe factory. A pair of monkeys then steal the car.
Alan rushes through town to find that it's in a bad state - stores all over are going out of business or are already bankrupt, there's graffiti and hobos everywhere, and the shoe factory is bust. A homeless man living there with his dog tells Parrish that after he disappeared, Sam and Carol put everything they had into finding their son until they eventually died - and when they went, so did the town’s economy. Just then, they see that the realtor who sold them the mansion was bitten by the mosquitoes from earlier, another of which descends upon them just as they hide inside a car. They escape back to the mansion, where Alan cleans up. He absolutely refuses to help the kids with Jumanji, but with some reverse psychology from Peter and finding his piece still on the board, Alan is forced to join in.
Realizing that they need Sarah Whittle's help, the three find Sarah's house, where she faints upon seeing Alan again. She had (been trying to) become a Psychic and has severe trauma from 1969 and thus refuses to play initially as well. However, Alan tricks her into rolling the dice, which summons huge plants - both large, yellow carnivorous pods (one of which nearly eats Peter) and poisonous purple flowers. Alan's roll next changes everything - it summons a big game hunter named Van Pelt out for Alan's head ever since his time in the jungle. Alan evades Van Pelt after the hunter runs out of shots, returning to the house where it's now Judy's turn, though Sarah and Alan are too busy arguing to notice. Judy's roll summons a stampede of rhinos, elephants, zebras and pelicans, and one pelican steals the game.
Alan tracks the pelican to a river, where one thing leads to another and Peter catches the game before it goes over a waterfall. Unfortunately, Carl tracks Alan down too and takes him in for questioning. During the scuffle, Peter tries to drop the dice so it would land on 12 to speed up and finish the game. The game counts this as cheating and punishes the poor kid by making him half-monkey. On their way to the police station, Alan reveals himself to Carl by apologizing to him for costing him his job, convincing Carl to let him go. Meanwhile, Van Pelt steals the game amidst the incoming stampede of animals and a mass looting started by the monkeys. Sarah, Peter and Judy chase him to a store and take it back.
The quartet return to the Parrish mansion (with its front somehow repaired from the stampede earlier) to find it completely overgrown with vines and leaves. Sarah rolls, which creates an indoor river, complete with a crocodile, which Alan wrestles and rescues Sarah from. A kick on the house's door by Bentley drains the water out, allowing them to return to the attic. The next roll by Alan turns the floor under him to quicksand, which is stopped with Judy's roll telling her to go back a turn. Peter's turn isn't any better, as it summons a group of big spiders. During their attempts to fight them off, one of the purple plants from earlier pops up and sprays Judy with barbs, causing her to fall ill, weakly wishing their parents were there. Sarah rolls one more time, which summons the worst hazard of all - an earthquake. As the house and land beneath them splits apart, Alan is freed from the attic floor and swings after the game, grabbing it before crashing through glass into the parlor.
Just then, Van Pelt arrives and demands Alan face him and drop the dice. He does so, with one landing on the board at "1" and the other rolling further into the house's ruins. However, instead of running from him again, Alan chooses to face Van Pelt head-on in spite of his fear. Right before Van Pelt can shoot Alan, the other die finally stops at "2". Alan sees his game piece reach the board's center, with the word "JUMANJI" signifying that he's won the game. Van Pelt fires, and Sarah jumps in front of Alan to protect him...but the bullet never makes it to Alan or Sarah. A whirlwind of African animals bursts through the house and picks Van Pelt up with it. As Sarah and Alan watch on, hold each other tightly and close their eyes, the tornado of trumpets, stripes and yelps for help completely sucks back into the green glass gem of Jumanji.
The next thing Alan and Sarah know, they're kids in 1969 again - everything that happened as a result of the game has been reversed, just as promised. The two embrace just in time to see the still-alive Sam walking back into the mansion to pick up his speech notes. Without thinking, Alan rushes to his father and tearfully apologizes for getting angry, with Sam doing the same. In response, Alan is told that he doesn't need to go to Cliffside if he doesn't want to, and Alan tells him that it was his fault for the factory incident, not Carl's.
As Alan and Sarah pledge to never forget Judy and Peter or what happens to them, the two get rid of the game by tossing it into a river, hearing its drumbeats sink into the water. Sarah then kisses Alan. By 1995, things are a LOT better - Alan and Sarah are married and expecting their first child, Carl still works with the factory, Sam and Carol are still alive and well, and Brantford is still bustling. At their Christmas party, they welcome the Shepherd family, including Judy and Peter and their parents, Jim and Martha - who are also alive and will stay that way after Alan and Sarah convince them to start working with them instead of taking the vacation that kills them.
All's well that ends well - except somewhere else in the world, where two French-speaking girls hear something while walking on a beach. Lying in the sand, Jumanji is waiting...