 An off-duty policeman is among the victims of a massacre on a town bus in San Francisco, and unearthing the person responsible becomes the task of SFPD Lieutenant Jake Martin, his parter Leo Larsen, and a squad of department detectives, who work to learn the identities of the varied victims in the hope that it will lead to clues to the killer. The investigation, however, repeatedly leads to dead ends, until the death of a young woman involved with one of the victims, Gus Niles, unearths a partnership between Niles and another man who has been investigated in the past. Martin, with the reluctant help of Leo Larsen, decides to smoke out the bus killer, a move that leaves him in danger when the killer has another bus in the sights of his sub-machine gun.