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The people of Minami-sakōjima Island, isolated far from Honshū Japan, were worrying over how to deal with depopulation. The key industry of this island is the aerospace industry owned by the Demizugawa Heavy Industry. Nevertheless Demizugawa Heavy Industry decided to close down the factories and laboratories on Minami-sakōjima Island. Takamidsuka senior high school is the only high school on Minami-sakōjima Island. This school has a dormitory named "Tsugumi Dormitory", which was remodeled from a former schoolhouse.

There's a collusive relationship between the principal of Takamidsuka senior high school and a resort development company "Rinchu Real-estate". Together they contrive to demolish the Tsugumi Dormitory and build a huge resort hotel on the lot. The principal hopes to tie into Rinchu Real-estate's interests. As a result he decided to stand for the Mayoralty of Minami-sakōjima Town.

The principal declares that he will close down the Tsugumi Dormitory if the number of boarders falls below five. Currently there are five boarders in this dormitory: Umi, Naoko, Miyaho, Shizu and Wataru. Next spring, Naoko will enter a University far from the island. Umi, Rinna, Miyaho and Shizu (Their fathers are engineers in Demizugawa) will move from Minami-sakōjima Island as well. Therefore Tsugumi Dormitory will be closed down and the boarders of Tsugumi Dormitory are destined to separate from each other. Together they spend community life as the last boarders of Tsugumi Dormitory.

A transfer student Rinna enters the Tsugumi Dormitory. She behaves disobediently in the school and the dormitory. She hates other boarders welcoming her. Wataru continues trying to welcome Rinna to the Tsugumi Dormitory. One day, Wataru noticed that Rinna is an excellent long-distance runner.