Monsters, Inc.

In a world inhabited entirely by monsters, the city of Monstropolis harnesses the screams of human children for energy. At the Monsters Incorporated factory, skilled monsters employed as "scarers" enter the human world through special doors that become portals to children's bedroom closets. They then frighten the children to harvest their screams; as human children are rumored to be toxic enough to kill a monster through physical contact, this work is considered dangerous. Furthermore, children are becoming harder to scare, causing a decline in production and subsequent energy crisis that worries Monsters, Inc. CEO Henry J. Waternoose III.

One evening after work, top scarer James P. "Sulley" Sullivan returns to the Scare Floor to retrieve forgotten paperwork for his best friend and assistant Mike Wazowski, when he discovers that an active door has been left at the station of his rival, Randall Boggs. Upon inspecting the door, Sulley accidentally lets a toddler girl into the factory. Frightened, Sulley unsuccessfully tries to return the girl, who escapes into Monstropolis, interrupting Mike on a date at a sushi restaurant in the process. Chaos erupts when other monsters see the girl, prompting Sulley and Mike to escape with her as the Child Detection Agency (CDA) arrives and quarantines the restaurant. Forced to keep the girl hidden in their apartment for the night, Sulley and Mike learn that she is not toxic and that her laughter generates more energy than her screams.

The next day, Sulley and Mike disguise the girl as a monster and attempt to send her home. While Mike seeks out her door, Sulley starts to grow attached to the girl and nicknames her "Boo". Randall, waiting in Boo's room to ambush her, kidnaps Mike by accident and reveals his plan to kidnap children and cruelly force screams from them using a machine called the Scream Extractor. Sulley and Boo rescue Mike, and they go to the simulator room to alert Waternoose of Randall's plot. There, Waternoose makes Sulley give a scare demonstration to some new recruits, and Boo becomes frightened.

While attempting to comfort her, Sulley notices images on the monitors depicting him scaring, making him question his line of work. Boo inadvertently reveals herself to Waternoose, who turns out to be in league with Randall; he abducts Boo and banishes Mike and Sulley to the Himalayas in the human world.

Mike and Sulley are found by the Abominable Snowman, who takes them to his lair. When he reveals a nearby village, Sulley hopes to use its closets to return to Monstropolis, but Mike, feeling betrayed, initially refuses to go with him. Sulley nonetheless returns to the factory and rescues Boo from the Scream Extractor. Mike returns to reconcile with Sulley, and Randall chases them through the enormous door vault as the trio searches for Boo's door. Randall eventually kidnaps Boo, but she overcomes her fear of him and helps Sulley defeat him; Mike and Sulley then banish Randall to the human world and destroy his corresponding door.

Just when Mike and Sulley locate Boo's door, Waternoose calls it down to the Scare Floor, intending to have the CDA arrest them. Mike distracts the CDA while Sulley and Boo lure Waternoose into the simulator room. Sulley tricks Waternoose into revealing his plan to kidnap hundreds of children to save the company and solve the energy crisis, while Mike records the conversation and replays it to the CDA, who promptly arrest Waternoose. Scare Floor administrator Roz then reveals herself as an undercover CDA agent, and allows Sulley to send Boo home, but orders her door shredded.

Some time later, Sulley retools the factory to harvest children's laughter rather than screams, since laughter is ten times more powerful. With the energy crisis solved, Sulley becomes the new CEO of Monsters Inc., while Mike becomes the company's top comedian. Mike then reveals to Sulley that he has painstakingly rebuilt Boo's door, allowing Sulley to reunite with Boo.