Spencer Gilpin is sent to detention for helping his former best friend, Fridge with his homework by writing his essays for him. They are joined by Bethany Walker, a girl who was caught talking on her phone during a quiz, and Martha, a girl who refused to be made to participate in gym class. For detention, they are charged by Principal with removing the staples from discarded magazines in an old storage area, but Spencer discovers the console containing the Jumanji game and convinces the others to play it with him. They start to pick characters. One of the five characters, however, is locked. Once they click start, they instantly get sucked into the game.
Finding themselves in a jungle, all four realize that they have become the avatars they chose for the game. The four freak out as they know they are in the game. Almost immediately, Bethany is suddenly eaten by a hippopotamus that emerges from the river, but she swiftly reappears after falling out of the sky.
Running from a stampede of hippos, the group encounter Nigel, whose repetitive responses help Spencer identify him as an NPC who tells them that they have come to Jumanji in response to a letter Nigel wrote to Spencer's avatar. The letter explains that Nigel was part of an expedition by explorer John Hardin Van Pelt to claim a legendary gem, the "Jaguar's Eye" if you take the gem from the large jaguar statue giving Van Pelt control over the animals of Jumanji. To complete the game, the players must return the gem to the jaguar statue and yell "Jumanji."
With those instructions, the group is dropped off outside a forest, where it is revealed that only Bethany can read the map Nigel gave them, the map directing them to the next level. They are then attacked by Van Pelt's bikers but manage to escape by fleeing through the trees and jumping off a cliff into a river. After getting out of the river, Martha realizes that she has been shot then explodes. Looking at series of line tattoos on their forearms, which initially consisted of three bars where Martha and Bethany now only have two, Spencer realizes that these bars indicate the number of lives they have in the game, guessing that they will die for good, both in the game, and in the real world as well, once they lose all three lives.
While traveling to the town where they will receive their next clue, Spencer loses one of his lives due to Fridge pushing him off a cliff. In the bazaar, Fridge loses one of his lives when he eats cake - previously realized as one of his character's weaknesses - and explodes, getting the attention of Van Pelt's minions. Fortunately, before they are attacked, they discover their next clue hidden in a basket containing a snake, which they can acquire by collecting their skills and coming together, allowing Spencer to catch the snake while Fridge defangs it. When Van Pelt's forces attack, Spencer attempts to fight them off, but they are soon confronted by Van Pelt himself, only just escaping when another player uses a smoke grenade as a distraction and leads them to a secret passage.
The fifth playable character introduces himself as the pilot Jefferson "Seaplane" McDonough, who says his real name is Alex. He leads the gang away. Alex takes the four to a treehouse that once belonged to Alan Parrish. He makes margaritas for the gang, and Fridge gets drunk. Alex mentions that he has been stuck in the game for months, and he hasn't been able to get to the transportation shed to get himself out. Alex has used up two of his lives and been unable to go past without the others due to his limited skills but has identified the transport shed where they will acquire the vehicles to move on. While Bethany teaches Martha in how to flirt to distract the guards, Alex's dated references lead Spencer and Fridge to realize that he is Alex Vreeke. Alex later realizes that he has been trapped in the game for more than 20 years. Although Alex suffers a panic attack, the team can gain access to the transport shed after Martha taps into her character's strength of dance combat, subsequently helping Alex recognize that they can complete the game by working together.
There is a brief panic when damage to the helicopter traps it in a canyon in front of a rampaging herd of white rhinoceros, but Spencer can repair the damage and help them ascend. Unfortunately, they lose the gem when it falls out of Fridge's backpack, but Spencer can reclaim the gem after using Fridge as a sacrifice to draw the rhino herd's attention, then telling Alex to rotate the helicopter just in time to catch Fridge as he respawns. Once they land, Alex is stung by a mosquito and begins to die due to his avatar's weakness, but Bethany saves him by giving him CPR, which passes one of her remaining lives to him. Alex.
Making their way to the jaguar statue, they find themselves confronted by actual jaguar guards, which cost Spencer one of his lives when he attempts to take the gem along the path to the statue alone. With Fridge's support, Spencer comes up with a plan where Martha takes out Van Pelt's approaching minions while the other three provide a distraction for the Jaguars, making Spencer take one of the minion's motorbikes and continue to the statue. Van Pelt attempts to gain the advantage by taking Bethany hostage, but Fridge can drive the jaguar guards away by riding an elephant. While the others distract Van Pelt's minions, Spencer takes the bike to the top of the statue while Martha takes the gem from a pit full of snakes. Using her character's weakness to venom to her advantage, she purposely dies to respawn and hand Spencer the Jem. With this action, Spencer places the gem in the statue and calls out Jumanji, Ending the game and restoring peace to the Jungle. With this victory, Nigel appears to congratulate the group, shaking their hands and returning them to the real world.
Back in the storage room, all four detention students have returned to their true forms, but are saddened to find that Alex is not among them. As they walk home, they witness the Vreeke household - which had fallen into disrepair as Alex's parents had no answer as to his fate - has been fully restored. As a car parks outside the house to release a family, the father sees the former players and confirms that he is Alex, who emerged from the game in 1996 and changed his history, also revealing that he named his eldest daughter after Bethany.