Secure within a desolate home as an unnatural threat terrorizes the world, the tenuous domestic order he has established with his wife and son is put to the ultimate test with the arrival of a desperate young family seeking refuge. Despite the best intentions of both families, paranoia and mistrust boil over as the horrors outside creep ever-closer, awakening something hidden and monstrous within him as he learns that the protection of his family comes at the cost of his soul.

A city of monsters with no humans called Monstropolis centers around the city's power company, Monsters, Inc. The lovable, confident, tough, furry blue behemoth-like giant monster named James P. Sullivan (better known as Sulley) and his wisecracking best friend, short, green cyclops monster Mike Wazowski, discover what happens when the real world interacts with theirs in the form of a 2-year-old baby girl dubbed "Boo," who accidentally sneaks into the monster world with Sulley one night. And now...