Plot
Monstropolis is a city populated entirely by monsters. The world can be connected to children's bedrooms in the human world through closet doors. When a door is properly activated, it becomes a portal between the monster world and the human world. The city's power supply is provided by Monsters, Inc., a power plant that employs monsters to scare children and extract energy from their screams. The company has a large warehouse full of doors, work areas called "scare floors" where the doors are activated, and a special training room in which employees practice their scare skills. The company's best scarer is James P. "Sulley" Sullivan (Goodman), whose assistant is his best friend and room mate Michael "Mike" Wazowski (Crystal). Sulley's main rival is Randall Boggs (Buscemi), but enjoys a patronizing relationship with the company's CEO, Henry J. Waternoose III (Coburn), who likes Sulley because of his scaring ability. Waternooses also worried about an energy crisis because children are harder to scare than they used to be.
One day, Sulley finds an activated door on his scare floor after the workday has ended. He finds no one in the room behind the door, but a little two-year-old girl (Gibbs) follows him back into the monster world. Far from being scared, she calls him "Kitty" and delights in playing with him. Since monsters think humans are lethally toxic, Sulley tries repeatedly to return the girl to her room, but she keeps following him back, and Randall eventually deactivates and stores the door, leaving the girl stuck in the monster world. The girl's presence becomes public knowledge after Sulley sneaks her into a restaurant to find Mike, so they hide her in their home while the Child Detection Agency (CDA) searches for her. Sulley decides to call the girl "Boo", and he slowly bonds with her after realizing that she is not poisonous. He also discovers that her laughter produces even more energy than her screams.
The next morning, Sulley and Mike disguise Boo in a monster costume and sneak her into work. Randall agrees to help them return her to her bedroom, but when Mike enters the room, Randall captures him in a box, believing he is Boo. Randall intends to kidnap Boo and subject her to a device that extracts her screams.
What follows is a sequence of battles, chases, and accidents in which Sulley and Mike attempt to protect Boo from Randall and his scream machine. Waternoose reveals that he is in cahoots with Randall and exiles Sulley and Mike to the Himalayas, where they meet the Abominable Snowman (John Ratzenberger). Sulley and Mike return to the monster world through a village at the foot of the mountain, where Randall chases them through the company's roller-coaster-like door-moving system. When the energy in Boo's laughter activates the doors in storage, the chase passes in and out of the human world. Finally, Sulley and Boo defeat Randall. Sulley throws Randall through the door of a trailer-park trailer, where a woman beats Randall with a shovel, and Mike destroys the door to make sure Randall never comes back.
Just as Sulley and Mike attempt to return Boo to her home, Waternoose and the CDA call her door to the scare floor, ready to arrest them, but Mike leads the agents away by fleeing with Boo's monster costume, and Sulley flees with Boo and the door. When Waternoose follows Sulley and Boo, Sulley attempts to set up and activate the door, and when Waternoose follows them through the door, he tells Sulley he is willing to kidnap children in order to save the company. However, Sulley had not properly activated Boo's door, causing the three to actually wind up in the adjacent Monsters, Inc. training room, which is equipped with a video monitoring system. Mike has recorded Waternoose's confession, and after he replays the confession, CDA agents arrest Waternoose.
With the scream-machine plot foiled, the CDA agents call in their leader, who is revealed to be Roz (Bob Peterson), the company's bookkeeper who was working undercover at Monsters Inc. Mike says goodbye to Boo and Sulley returns her to her bedroom, then Roz has the door shredded, preventing monsters from ever visiting Boo again. Sulley keeps one of the wood splinters as a memento.
Some time later, Sulley becomes the CEO of Monsters, Inc., and the company has ended the energy crisis with his policy of making children laugh instead of scaring them. Meanwhile, Mike has collected and reassembled the pieces of Boo's shredded door. When Sulley puts his piece in its place, the door is activated again, and when he peeks into Boo's room, she greets him.
