When an evil spirit known as Pitch lays down the gauntlet to take over the world, the immortal Guardians must join forces for the first time to protect the hopes, beliefs and imagination of children all over the world.

Susan Murphy (a.k.a. Ginormica) and the Monsters are now working with the US government as special ops. So when an alien presence is detected in Susan's hometown of Modesto, California -- right before Halloween -- the team is dispatched to investigate. Everything appears normal, right down to the jack-o-lanterns peering out from every doorstep and windowsill. But when Halloween arrives, those innocent-looking carved pumpkins reveal themselves for what they really are mutant aliens. The altered pumpkins then start to implement their fiendish plan to take over Earth. The Monsters are there to combat the mutant gourds and try to smash their wicked scheme!