Dai Byoin Senkyo Mae the Night Before

Half a year before the occupation incident. Koichi struggles to discover the truth Kaiseido Hospital hides. Meanwhile, Koichi and Sagami infiltrate Tajima Design Office, which designed Kaiseido Hospital. They're looking for blueprint data, but suddenly three employees and a security guard show up. It seems that they were contacted by the police asking them to come because they wanted to hear about the theft. However, the elevator that everyone got on stops on the way. Furthermore, a mysterious gas blows out and consciousness is cut off. When they woke up, they heard a mysterious voice telling them that a black object had been wrapped around the necks of the employees, and that a poisonous injection had been placed in their collars. Amidst a sudden sense of urgency, the voice added: "If you want to get out of the elevator alive, find the killer inside."
(Source: Hulu)