A young prince from Monster Land named Tarou Kaibutsu, known to his friends as Kaibutsu-kun, travels to the human realm to prove his worth as the future king. He takes up residence in a haunted house in Tokyo, accompanied by his three loyal monster servants: Dracula, a vampire who drinks tomato juice instead of blood and takes pride in being the group's intellectual; Wolfman, a short middle-aged man who transforms into a wolf when he sees a full moon or any round object and serves as the group's cook; and Franken, a large, incredibly strong but gentle creature who communicates through grunts that his companions can understand. Despite looking like a normal boy, Kaibutsu-kun possesses the ability to stretch his limbs like rubber and change his face by rubbing it rapidly, though he has a notable fear of lightning and hides during thunderstorms.

In the human world, the monster prince befriends Hiroshi Ichikawa, an orphaned boy who lives in a small apartment with his older sister Utako. Hiroshi often finds himself caught in supernatural incidents, leading him to call upon his unusual friend for help. The series follows Kaibutsu-kun and his companions as they encounter various other monsters who have come to the human world, ranging from troublesome troublemakers to those simply struggling to adapt to human society. Many of these encounters involve assassins and agents from the demon group known as Demonish, who oppose the young prince and his mission. The prince is also occasionally visited by his father, the King of Monster Land, and interacts with a recurring rival named Demokin, the prince of the demons.

Originally serialized in Shonen Gaho and Shonen King magazines from 1965 to 1969 before a revival run in CoroCoro Comic from 1980 to 1982, the manga follows a comedic episodic structure where Kaibutsu-kun must balance his royal duties and selfish, short-tempered personality with genuine care for his human friends and even his bumbling servants. The human world setting of Tokyo provides a backdrop for monster-of-the-week style stories, while flashbacks or journeys to Monster Land occasionally explore the prince's origins and the broader supernatural world he comes from. A notable narrative thread involves the prince being sent to the human realm specifically for ascetic training, with the implication that he must learn about humanity and compassion before he can truly earn his crown.

A separate manga titled Kaibutsu Shounen, written and illustrated by Arata Yamaji, was published in 2022 across two volumes. This series centers on a boy named Aka Arima who gains the power of a monster in exchange for his body and fights against dangerous demons that have overrun the world.