Fun-loving California girl Susan Murphy is struck by a meteorite covered in extra-terrestrial goo and mysteriously grows in size until she stands an impressive 49 feet and 11 inches tall. Placed under armed guard and held hostage by the military in a secret government compound, Susan learns other so-called monsters have suffered a similar fate. Her fellow captives include brilliant Dr Cockroach PhD, half-ape half-fish The Missing Link, indestructible gelatinous mass B.O.B. and a 350-feet long grub called Insectosaurus. When a giant alien robot lands on Earth and begins to run amok, repelling all military hardware, President Hathaway reluctantly issues an executive order to free the monsters and set them upon the robot, under the aegis of General W.R. Monger.