Taro Kaibutsu (怪物太郎, Kaibutsu Tarō), widely known as Kaibutsu-kun, serves as the prince of Monster Land and central protagonist in all adaptations. On his scheduled coronation day, his father, the King of Monster Land (怪物大王, Kaibutsu Daiō), commands him to undergo ascetic training in the human world. This decree sends Taro to the Human Realm alongside three attendants: Count Dracula, who substitutes tomato juice for blood and suffixes speech with "damasu"; Wolfman, the group's housekeeper and cook who ends phrases with "gansu"; and the intellectually limited but physically imposing Franken.

Taro wields metamorphosis abilities to overcome challenges during his primary mission: battling monsters threatening inter-realm peace, particularly assassins from the demonic organization Demonish. This group, led by the world-domination-seeking Demorina, aims to manipulate the demon prince and destabilize both realms.

Residing in the modest Arama-sō apartment complex within the human world, Taro bonds closely with orphaned siblings Hiroshi and Utako Ichikawa. Hiroshi becomes his chief human confidant, while Utako adopts a caretaker role, anchoring Taro's human experiences.

Occasionally, the King influences events remotely from Monster Land. A romantic subplot involves Taro's girlfriend, Kaiko-chan, though her presence remains secondary to the core conflict and training narrative.

The franchise spans a 1968–1969 anime, a 1980–1982 reboot, a 2010 live-action drama, and a 2011 3D film. While core traits and Taro’s mission persist across adaptations, settings and supporting cast vary. The live-action series introduces figures like Wolfman’s daughter and omits certain anime-exclusive classmates, yet consistently depicts Taro combating demonic forces and evolving through human connections. The 2011 film Kaibutsu-kun: The Movie achieved notable commercial success in Japan.