2030, the boundaries of space are officially opened. Visitors from different planets in the system are allowed to come to Earth. Japanese government decides to limit galactic trade to the artificial island of Dejima created off Okinawa but the encounter of species from other worlds creates tensions on the island. A special force, made up of humans and aliens, is created to maintain order in Dejima: the Dragon Gun. During a patrol, DG officer Σ (Sigma) casually stops a lorry carrying illegal goods. Inside is a Mumère, a species coming from a marine planet, which ends its life to protect a heavy secret. This will be the beginning of a long investigation for the officer Σ.