Doraemon suggests going on an underwater adventure for a summer vacation, and they find a underwater civilization that doesn't have contact with surface nations.

Nobita and his friends are arguing about whether to go camping in the sea or the mountains so Doraemon proposes to them that we go to the mountains under the sea. When they gain abilities that be able to adapt themselves to every kind of environments by Doraemon's tool they drive the water buggy into the deep sea for adventure.—tama