March 20, 1995. As Tsurugi Tatsuhiko, head of the hospital's emergency center, walks down a deserted corridor before the start of treatment, nurse Hoshino Nao comes up behind him and starts talking about dealing with late-night emergencies. At the same time, at Kita-Senju Station on the Hibiya Line, Sonoda Naoki, the driver of the A20S train from Kita-Senju to Nakameguro, gets into the driver's seat and performs his usual duties, such as inspections. With Sonoda at the wheel, the train starts moving. However, the passengers gradually start coughing, and some even become ill. One of the passengers presses the emergency button, and the train stops at the next station, Tsukiji, for a check. For some reason, the passengers' feet are wet... When the train arrives at Tsukiji Station and the doors open, passengers who are feeling unwell tumble out onto the platform. When Sonoda came out of the driver's cab, he was shocked by the sight before his eyes...