It is Halloween in Joliville, and the streets are filled with children in costumes, but the Monster Agency faces a unique crisis when real monsters decide to join the festivities. Because everyone assumes the creatures are just people in high-quality costumes, the monsters are able to cause chaos without being detected by the public. Matt, Manson, Dink, and Bruce must find a way to track down and capture the real threats hidden among the trick-or-treaters. The mission becomes even more complicated as the team tries to save the holiday without causing a mass panic that would ruin the celebrations for the entire town.