We see the adventures of the little girl Boo, Sully and Mike as they work out a way to get her back to her bedroom and along the way find a conspiracy that goes all the way to the top of Monsters Inc. They also discover that it is better to capture laughter than screams.

"Monsters, Inc." is a comedy set in the realm of things that go bump in the night, where chaos breaks loose after a hapless monster accidentally lets a human child into the secret world." A comedy, eh? Bascially, the film is about a little girl who enters the world beneath her bed, a world inhabited by monsters.

Monsters Inc. is about a parallel world unbeknownst to human beings called Monsters, Incorporated, a world of monsters who creep up under your bed and spring out from closets when your parents aren't looking. The monsters live by the screams and fear that they get out of children. With the advent of the new century, jaded children aren't that easy to scare anymore, so this world is in jeopardy, experiencing their version of an energy crisis, if you may. In comes our kind hearted hero monster, who simply wants to help his fellow beings out. One night, monsters Mike and Sully are doing their jobs, scaring kids, when they run into a kid who isn't scared of them at all. In fact, she's been waiting for the monsters to show up all night. She's always been obsessed with the monsters, and wants to be one of them, and go around the world scaring people. She's even made her own monster suit, and wants to be called just by her monster name "Boo". Boo follows Mike and Sully back to Monster World, where she convinces them to train her how to be a good scarer. At first they don't want anything to do with her, but pretty soon they soften up and let her go to work with them in disguise. They have to keep her identity as a kid secret, though, or else their boss, who is a big crab (literally) will fire them for letting a human into Monster World. There are a lot of close calls where Boo almost gets found out. Along the way Boo meets some of the legendary monsters, like the Lock Ness Monster, Bigfoot, the Abominable Snowman, and the Sea Serpent. She gets pretty good at frightening kids, and starts to make a name for herself. Mike and Sully are proud of their new "trainee". One night she gets assigned to scare a kid that was a neighbor of hers in the Human World, and he recognizes her and blows her cover. There's an uproar in Monster World that one of the best scarers in the business is really a human kid. A mob of monsters comes after Boo, and for the first time in her life, Boo gets really scared. She sees that being scared really isn't a cool thing, and realizes it was wrong for her to scare other kids. She talks to the monsters about it. They realize she's right. They've been profiting off of the misery of others all this time. Even if they're just humans, they still have feelings. Boo goes back to her Human World life, happy for the experience, but she's learned a lot about compassion.

In the Monsters, Incorporated, the largest scare factory in the monster world, James P. Sullivan (John Goodman) is one of its top Scarers. Sullivan is a huge, intimidating monster with blue fur, large purple spots, and horns. His Scare Assistant, best friend and roommate is Mike Wzowski (Billy Crystal), a green, opinionated, feisty little one-eyed monster. The cast of characters also includes the factory's CEO, a crablike monster named Henry J. Waternoose (James Coburn) and the snake-headed, beguiling receptionist Celia (Jennifer Tilly). One of the top Scarers is a sarcastic chameleon monster named Randall Boggs (Steve Buscemi). Visiting from the human world is Boo (Mary Gibbs), a tiny girl who goes where no human has ever gone before.