Satsuki, a boy high school boy and his childhood friend are riding in an underground train until it suddenly stops. Upon closer inspection the denizens of the train discover a mysterious beast! Panic sets in as the passengers struggle to survive the beast and escape to the surface.

After a tragic accident at a large facility 6000 meters below water killed its workers, the facility was abandoned for three years. After the Japanese company owning it is taken over by a Chinese company, the facility will be opened again. Kadokura Kengo and his colleague Danzaki are sent along with the team reopening the underwater facility. However, right away, an 'accident' grievously injures Danzaki. Kengo is left alone with colleagues he doesn't trust in an eerie underwater environment that may have more to it than meets the eye...