Mannix becomes involved in murder and political intrigue when he helps newspaper publisher Jason Walker finds out the reasons for the strange behavior of his daughter Diana. The case rekindles a romance between Mannix and Diana, as he tries to prove that she is being framed in the hit-and-run death of an underworld figure.

Mannix works with a police detective to find the reasons behind the death of Peter Martin. When Mannix accuses Carlos Valera, the private eye takes on more trouble than he bargained for. After Valera's arrest, Mannix begins to have doubts about his guilt and begins digging further into the case despite hostility from Valera's neighbors as well as Martin's sister.