When Mrs. Tremayne is mysteriously poisoned with gas, ambulance driver Frank Jessup meets her refined but sensuous stepdaughter Diane, who quickly pursues and infatuates him. Under Diane's seductive influence, Frank is soon the Tremayne chauffeur; but he begins to suspect danger under her surface sweetness. When he shows signs of pulling away, Diane schemes to get him in so deep he'll never get out.