A pair of bizarre murders occur wherein the victims are drained of their rare Type One blood type. Reporter Walter Garrett (Wayne Morris) consults with his friend Dr. Mike Rhodes (Dennis Morgan) which leads them to Rhodes' former mentor, hematologist Dr. Francis Flegg (John Litel). Flegg is initially unhelpful, but when Garrett and Rhodes notice a striking resemblance between Flegg's strange assistant, Marshall Quesne (Humphrey Bogart) and the late Dr. Maurice Xavier, they confront Flegg. Flegg admits that he used scientific methods to bring Xavier back from the grave and employed a synthetic blood formula to sustain his life. However, the formula is unstable, and therefore, Quesne/Xavier must seek out human victims with the rare Type One blood type contained in the formula in order to stay alive.
A hunt is begun for Quesne, who has discovered that Joan Vance (Rosemary Lane), a nurse and Rhodes' sweetheart, is a carrier of the rare blood type. He escapes with her in a taxi, professing to be taking her to Rhodes. Barnett and Rhodes, accompanied by the police, track them to their location. Quesne is shot dead, and Joan is saved from the fate of the others.
Who track down Quesne and Joan?
Barnett and Rhodes, accompanied by the police