"I'll be your leg, Master!" 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… It's as if she is a piece of scrap. Atri seems to have received a very important order from Nonko and tries to remember what it is, but...

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. There, Natsuki is faced with the saddening reality of the lives of the people who remained in their town.

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. As soon as he gets back he gets to work. As they are working, Natsuki and Ryuuji exchange heated opinions over how the generator should be built. Atri intervenes and the two guys become closer friends. Meanwhile, a suspicious-looking character peeks at them from outside of the school....