On Doraemon’s birthday, Nobita wants to give him a special surprise gift and becomes interested in finding a rumored rainbow-colored butterfly. After returning home disappointed by Gian and Suneo’s prank, he discovers Doraemon repairing a machine that can travel underground. Suddenly, a mysterious masked girl named Lapis appears and drags Nobita and Doraemon into an underground treasure hunt, claiming to be a genius scientist from space. Meanwhile, Dorami learns that Doraemon may have been kidnapped by a dangerous criminal and teams up with the future police to track them down. The adventure soon turns into a thrilling underground mystery involving hidden treasure, the rainbow butterfly, and Lapis’s true identity.