The story of Water Brain takes place in a fantastic steaming city with oriental charm. Behind every child in the city, there is a huge monster urging them to do endless homework. The back of their head is a container filled with water, which will expand due to tension, producing the monster's food-pressure marshmallows. After the monster eats it, it will generate energy, which will be collected by the collector and supplied to the whole city.

Xiaole, the protagonist, is an imaginative child in this city. By chance, the child discovered that paper airplanes can make monsters smaller, and the classroom began to be chaotic. In the chaos, the child got unprecedented happiness, but after the classroom alarm sounded, the child was suppressed. After school, the children took up the monster and left, because the monster was their heavy schoolbag. Xiaole pinned his hope for freedom on a hidden paper plane. He vigorously flew the paper plane out under the obstruction of the monster. The paper plane soared freely and flew to a beautiful sky that children had never seen before, but at the same time, the paper plane began to burn. ...