Nobita insists dinosaurs still survive, and even Doraemon's gadgets say he is wrong, until Suneo spots a huge one in the river. With a gadget that opens a tunnel into a vast cavern, Doraemon gives the friends a secret underground playground, but Suneo wanders off and does not come back. Searching for him, the others find a hidden world beneath the surface, where Banhou, a knight who rides a dinosaur, saves them from the kappa who live there and tells them Suneo is safe in the capital, Enriru.