When Susan Murphy is clobbered by a meteor of outer space gunk she mysteriously grows to 49-feet-11-inches tall.
Alerted to the threat of this new monster, the military jumps into action and Susan is captured and secreted away to a covert government compound.
There, she is renamed Ginormica and placed in confinement with a ragtag group of other monsters: the brilliant but insect-headed Dr. Cockroach, Ph.D.; the macho half-ape, half-fish The Missing Link; the gelatinous and indestructible B.O.B.; and the 350-foot grub called Insectosaurus.
However, when a mysterious alien robot lands on Earth and begins storming the country.
The President, in a moment of desperation, is persuaded by General W.R. Monger to enlist the motley crew of Monsters to combat the Alien Robot and save the world from imminent destruction.