The Last Kids on Earth and the Monster Dimension is the ninth book in a long-running series of middle grade novels following Jack Sullivan and his friends as they confront zombies and monsters in a postapocalyptic world. The first book in the series, The Last Kids on Earth, begins during the apocalypse, with Jack living alone in a treehouse fortress that he constructed. Jack is an orphan, and his foster family promptly abandoned him when the apocalypse began. As a child in the foster care system, bounced from family to family, Jack knows how to take care of himself. He eventually finds his best friend, Quint; his former bully, Dirk; and his school crush, June, and convinces them to join him in the treehouse fortress, where they fight zombies and monsters and survive together.

Throughout the series, Jack and his crew face new monsters and piece together the greater cosmic conflict facing Earth. They face zombie foes, human villains, and new monster threats and must eventually leave the treehouse to fight in the cosmic battle. Jack accumulates new supernatural powers over the course of the novels, first with his baseball bat, then his Louisville Slicer, and then his Cosmic Hand, a severed monster tentacle that has merged with his arm, allowing him to telepathically control zombies.