The quiet life of the cheerful hippopotamus Mumbo is turned upside down: suddenly he is gigantic! That is less fun than it sounds, because now he no longer fits in the house with his mama... There is clearly magic at play, so Mumbo, together with his three best friends, goes in search of a mysterious witch who can make him small again. On the outside, anyway, because on the inside the four of them keep growing to the fullest.