The story of Water Brain takes place in a fantastic steaming city full of oriental charm. Behind every child in the city, there is a huge monster urging them to do endless homework. The back of their heads is a container filled with water, which will expand due to tension, producing food pressure marshmallows for monsters. The monster will produce energy after eating, which will be collected by the collector and supplied to the whole city.

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