In the near future, much of the world has been forgotten at the bottom of the ocean. Natsuki Ikaruga is invited by a mysterious woman to search for a treasure in his grandmother's house. One day, he dives to the bottom of the ocean in a submarine and encounters a mysterious capsule with a girl sleeping inside. The girl was a humanoid so sophisticated that she could be mistaken for a human.

Atri, who was supposed to be with Cathrine, has come back to Natsuki. Although she was pumped to be of assistance to her new master, she fell into the ocean while carrying him and failed at cooking.

Natsuki accompanies Atri who is on her way to school. However, when they arrive, they find that most of the school is underwater. Although they managed to get into the school building, inside it was full of empty classrooms and an unexpected teacher.

Natsuki sets his heart on creating a tidal power generator that utilizes the ebb and flow of the ocean's tides. He dives back into the ocean to find some materials and salvages some resin from the ruins of Ryuuji's family's factory.

Atri, who was supposed to be with Cathrine, has come back to Natsuki. Although she was pumped to be of assistance to her new master, she fell into the ocean while carrying him and failed at cooking.

Natsuki's life experiences a big shift after becoming a teacher and starting a salvaging service. Rumors of a school lit with electricity attracts new students, and activities around the school regain a sense of liveliness.