Monsters generate their city's power by scaring children, but they are terribly afraid themselves of being infected by children, so when one enters Monstropolis, top scarer Sulley finds his world disrupted.
The story centers around the workers at Monsters, Inc., the power plant of the monster world The plant employs scarers who generate power from the screams of little children. James P. Sullivan, or Sully to his friends, is the company's best man, popping out of closets and generating the most screams per volume. He is aided by Mike Wazowski, a nervous, little one-eyed creature whose main concern is winning the best scarer contest. Their only competition is a weasely, backstabbing co-worker named Randall. Things are going well until one night when a human child wanders through a neglected door and attaches herself to Sully. Sully is beside himself with fear for himself – the monsters have been told that they'll die if touched by a human – and what will happen to the company if this breach is discovered. Desperate for help, he interrupts Mike's date with his girlfriend Celia, where the girl gets loose and wreaks havoc on the restaurant patrons just by walking around.
Mike and Sully are initially horrified to be stuck with this disgusting creature, but she soon wins Sully's heart with her adorable and loving behavior. He becomes so enraptured with her, he begins to call her Boo. They know that Randall is up to no good, but they think that he's just cheating to win the contest. They have no idea that the real reason behind his leaving Boo's door open is much more evil than trying to beat them for an honorary company title. Though not the sharpest knife in the drawer it becomes apparent to Sully that Randall's intentions towards Boo do not have her well-being in mind. He refuses to go along with Mike's plan to send her back, regardless of what will happen to her. Attempting to foil Randall's scheme soon gets them in more trouble than they ever imagined and Boo in the hot seat, fighting for her life. Of course, she has Mike and Sully to fight for her and in the end, they foil the evil plot, figure out a new way to generate energy that solves the power crisis and make a new friend for life.