It's Halloween night and 10-year-old Mona is stuck inside without so much as a scary movie to watch. She figures this Halloween couldn't get any worse, but a giant monster appears in her living room to prove her very, very wrong. She is chased out into the night where she encounters some odd new friends and finds herself on a quest to save humanity from billions of monsters. Though Mona is the textbook definition of a reluctant hero, she and her friends will do their best to keep this from being... The Last Halloween.