Kaibutsu-kun, prince of Monster Land (Kaibutsu Land), travels to Tokyo accompanied by his three loyal attendants: Dracula, who drinks tomato juice and appends "damasu" to his sentences; Wolfman, the cook and housekeeper who appends "gansu" and transforms under full moons; and Franken, a strong but largely nonverbal member who communicates with guttural sounds like "Unga". As royalty, Kaibutsu-kun commands his companions with a spoiled demeanor. Despite resembling a human boy, he possesses supernatural abilities: stretching his limbs for pursuit or intimidation and rapidly altering his facial features to mimic others for disguise or locating individuals. He harbors a significant fear of lightning, driving him to seek shelter during storms.

In the human world, Kaibutsu-kun resides in a haunted house and forges a close friendship with Hiroshi Ichikawa, an orphan living with his older sister, Utako. Hiroshi frequently encounters monsters from Monster Land causing disturbances or struggling to adapt, prompting Kaibutsu-kun to intervene. His relationship with his father, the King of Monster Land (Kaibutsu Daiō), becomes critical when the King falls ill. This motivates Kaibutsu-kun to invite Hiroshi back to Monster Land, where Hiroshi is turned to stone after encountering the King. Kaibutsu-kun and his friends then race to reverse the petrification. He consistently confronts the antagonistic demon group Demonish and its leader, Prince Demokin, who seek to disrupt harmony between worlds, thwarting their assassination attempts and schemes.