Unknown to his wife, Professor Brenner, who teaches criminal psychology at a New York university, leads a double life as Karl Wagner, who runs a mission in the Bowery at night. Brenner's assistant at the mission, Judy Malvern, is also unaware of her employer's secret life and the fact that behind his office is a passage leading into subterranean rooms. Brenner uses the mission as a front as he masterminds robberies executed by hired thieves such as Stratton. One night, Brenner, Stratton and escaped convict Fingers Dolan rob a jewelry store and Brenner forces Stratton to kill Fingers. When killer Frankie Mills uses the mission as a hideout, Brenner takes him in and gets the ruthless young man to kill Stratton. Brenner, who had murdered other minions, orders his cohort, drug addict Doc Brooks, to bury them in the basement. When Brenner insults Brooks, the irrational doctor decides to prove his qualifications and secretly revives the dead criminals. In the meantime, Judy's fiancé, wealthy student Richard Dennison, fails to convince her to give up her work at the mission. He then decides to write a term paper for Brenner's class on the psychology of the underprivileged. Dennison, in tramps' clothing, goes to the mission and is shocked when he recognizes his professor posing as "Wagner." Brenner, who has just completed another robbery and murder, pretends to take Dennison in confidence, but actually has Mills kill the student. Policeman Pete Crawford is sent to investigate Dennison's disappearance and his search leads him to the mission, where he consults with Judy. He and his partner later go to Brenner's home, where they recognize a picture of Brenner as "Wagner." Mrs. Brenner finally comprehends her husband's late night absences and plans on going to police headquarters with Pete, but Brenner hides in the bedroom and silently murders her and escapes. At the mission, Judy promises to get Brooks his next drug fix if he shows her the secret rooms, but Brenner intervenes and orders Mills to kill Judy. Mills refuses to kill a woman, and as police, led by Pete, flood the mission, Brenner and Mills attempt to escape. Mills is fatally shot and Brooks, leading Brenner ostensibly to safety, takes him into his own secret chamber, where the revived victims attack their killer. Dennison fully recovers and he and Judy plan to marry.