酷似河童的男孩・三平，因意外的事件误入水底世界，被河童抓住。但是却和自己长得一模一样的河童，加太郎成为朋友，顺利地逃到了河童的国家。回到人间界的三平为了寻找失踪的母亲，和加太郎、淘气狸的黑麿、小米瓦拉、堕落死神等一起，去了住着可怕妖怪的鬼首岳。因为人的隧道工程的声音而患失眠症的一角鬼，为了听摇篮曲而把母亲带走了。由于地基崩塌，三平等人差点被压在下面，一角鬼拼命地救了他们。就这样，母亲和离家出走回来的父亲三平三人顺利地相见了...

Adapted from Shigeru Mizuki's classic manga from the 1960s, 'Kappa no Sanpei', the anime tells a story of a young boy, Sanpei, who lives in the beautiful countryside with his grandfather. One day, he is caught in the muddy streams of the river and wakes up in the land of goblins. Sanpei makes friends with a goblin named Gartaro, who bears striking resemblance to himself. Thus, the adventure begins.