The story is set in the year 311 of the Orbital Calendar, when humanity has migrated to countless colony clusters in space. A space colony girl named Akiha Shishidou encounters a malevolent artificial intelligence named Leopard that has been installed on a colony.

Akiha is joined by an Inter-Colony Police officer named Itsuki Kannagi, a taciturn young girl named Honoka Kawai, and a robot named Imoko "Imo-chan" Shishidou.