When California resident Susan Murphy of Modesto, California grows to 49 ft 11 in (because of being hit by a meteor while being late for her wedding), she is considered a monster and captured by the military. She is remanded to an institution that houses other monsters: Dr. Cockroach, Ph.D., an insect-headed mad scientist, the Missing Link, a 20,000-year-old fish-man, B.O.B., a gelatin-like, jolly, indestructible monster and Insectosaurus, a fuzzy bug that stands at 350 feet tall, making even Susan look tiny by comparison. When a UFO lands and an alien overlord named Gallaxhar demands that Earth be given to them, General W.R. Monger suggests to the President that the monsters can provide the firepower to defeat the aliens.