In the dead of night, in the remote wilderness of California, a huge flying saucer appeared over the pumpkin field of old farmer Jeb. The flying saucer stretched out two pipes, poured mysterious liquid into the field, and flew away from the earth. The old farmer who heard the sound came out to check, but encountered the most horrific scene. Meanwhile, at the secret military base, Susan Murphy (voiced by Reese Witherspoon) is discussing how to spend a fun Halloween with the Lost Monster (voiced by Will Arnett), BOB (voiced by Seth Rogen), and Dr. Mantis (voiced by Hugh Laurie), but General Mungo (voiced by Kiefer Sutherland) has given them the task of investigating UFO incidents. On Halloween night, the monster companions had fun on the street, but the terrifying pumpkin monster wanted to turn this place into a real nightmare...