On the day of her wedding, Susan Murphy is hit by a meteor, which causes her to grow to gigantic size. She is captured by the military and held in a secret facility underground. There, she meets other monsters, like half-man, half-fish The Missing Link; an indestructible blob named B.O.B.; brilliant scientist Dr. Cockroach; and a giant grub named Insectosaurus. When an alien attacks Earth, the government decides to turn the monsters loose to fight the aliens.