"Dad? What's happening?" the young prince of the Kaibutsu or Monster Kingdom asks his father. But before he knows it, the king has thrust him into the human world. Prince Taro (Satoshi Ohno), also known as Kaibutsu-kun must live among people for a while as part of a grooming process before taking over the kingdom. So begins the sometimes scary, sometimes touching and often very funny adventures of Kaibutsu-kun and his three affable attendants Dracula (Norito Yashima), Wolfman (Ryuhei Ueshima) and Franken (Choi Hong-Man).

The monsters meet humans Hiroshi (Tatsuomi Hamada) and his older sister Utako (Umika Kawashima), and through them, they learn about friendship, love, family, and ultimately, the purpose of their mission: what it takes to be a leader.

Kaibutsu-kun is the dramatization of a classic and beloved Japanese manga from the 1960s. In this adaptation, the prince will be played by Satoshi Ohno, leader of the pop idol group ARASHI.

怪物王国的王子怪物君怪物太郎（大野智饰）是任意妄为的浪荡子。见其任性的行径，怪物国王（鹿贺丈史饰 ）命令儿子怪物君到人间界修行，直到成为名副其实的国王为止。作为陪伴德拉克拉（八岛智人饰 ）、狼人（上岛龙兵饰）、佛兰克（崔洪万饰）3人一同前往。「要做对人类有帮助的事情」，得不到国王允许就不能回怪物王国。而且将胳膊拉长，从口中吐出火焰等法力都被封住，还有如果在人间界怪物的真正面目被暴露，就永远回不去怪物王国…

「为什么我要遭这种罪！」怪物君非常生气，说完就把国王送的发出怪异之光「非常有用」的魔王石抛了出去。偶然间总被欺负的小学生宏（滨田龙臣饰 ）拾到那块魔王石。怪物君将欺负宏的孩子们教训了一顿，但其实宏是因为说谎，被大家所排斥。

同时，与怪物大王敌对的恶魔界干部恶魔丽娜（稻森泉饰）派阿库玛（津田宽治饰）到人间。阿库玛接近宏，将宏的欲望全部吸进，企图将恶魔王子恶魔金苏醒。怪物君在宏的姐姐歌子（川岛海荷饰）的请求下前去拯救宏，与阿库玛相持。可是千钧一发之际怪物君的法力却使不出来，大家处于危机局面…？！

自1965年开始连载以来，曾两度搬上电视动画版（1968年、1980年）以及两度搬上电影版（1981年、1982年）的藤子不二雄Ⓐ的代表作「怪物君」，此次以真人版首次搬上荧屏。