夜深人静时分,地处偏远的加利福尼亚荒郊野岭,一只巨大的飞碟出现在老农夫杰布的南瓜田上空,飞碟伸出两个管子,向田里倾倒了神秘的液体之后飞离地球。听到声响的老农夫出来查看,却遭遇最为恐怖的景象。与此同时,在秘密的军方基地里,苏珊·墨菲(瑞茜·威瑟斯彭 Reese Witherspoon 配音)正和失落怪(威尔·阿尼特 Will Arnett 配音)、BOB(Seth Rogen 配音)、螳螂博士(Hugh Laurie 配音)正在讨论如何度过一个好玩的万圣节,但是蒙戈将军(Kiefer Sutherland 配音)却交给他们一项调查UFO事件的任务。 万圣节当夜,怪物伙伴们在街道上玩得不亦乐乎,然而恐怖的南瓜怪却要将这里变成真正的噩梦……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!