The story continues after the events of "Monsters vs. Aliens," as Susan Murphy (Witherspoon) and the monsters now work with the US government as special operatives. 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 are revealed for what they really are - mutant aliens. The altered pumpkins then start to implement their fiendish plan to take over Earth. The mosters are there to combat the mutant gourds and try to smash their wicked scheme.