The plan is temporarily foiled when Nobita's mom tells him to go finish his homework, but when the Bermuda Triangle is on the news, Gian and Suneo decide the join the other three.

They find the treasure ship mentioned on the news, Nobita, who is outside, sees a strange fish shaped craft that started attacking him with an energy weapon.

That night, Buggy wakes Shizuka and reveals he saw what happened with Gian and Suneo when they lost consciousness, showing his memory like a movie projector.

Soon, everyone else watches the footage as well, seeing these "sea people" saving Gian and Suneo with their own Adaptation Light, before the craft Nobita saw appears and starts attacking them.

Nobita, Doraemon, and Shizuka later wakes up in a room where they meet one of the sea man called Eru, revealed to be inhabitants of the underwater kingdom of Mu.

While Gian and Suneo are put in a cell due to their personalities, the group rescues them and flee the kingdom, with Eru realizing their escape and giving chase.

After bypassing a fleet of fish craft and the barrier that leads into the kingdom with Doraemon's hat, they went on to search the "Devil's Castle", where Atlantis' main computer, Poseidon resides.