A boy from a standard school accidentally walks through a dynamic, shimmering portal in a hallway, leading him into the chaotic gymnasium of a monstrous alternate-dimension school. In this new world, he watches a "monster boy"—a student with distinct monster traits like grey skin and small horns, named Kai—fight an overwhelming swarm of seventy-eight diverse and bizarre creatures, ranging from winged imps and goblin-like figures to shadow monsters and sentient blobs. The action is frantic, with Kai utilizing dual energy blades, surrounded by the massive horde on a cracked gym floor, all under the curious and chaotic eyes of other observing monster students.