Throughout the United States, executions of convicted rapists and serial killers are conducted by a mysterious figure known simply as MR. X. Mr. X travels from prison to prison in complete anonymity. He dresses entirely in black and wears a mask to hid his face. No one knows who he is or where he lives, and he is always paid in cash. Tonight he arrives at the Idyllwind Penitentiary in the California desert. There, three killers will be executed over the next three days.

Presiding over the penitentiary is EARL W. CHENEY, the pig-headed, publicity starved warden. His deputies are WANDA, a struggling single mother, and HANK, a half-witted drunk who lets Wanda do all the work. In a meeting with the newly arrived Mr. X, the ground rules are laid down. Executions are performed at dawn. During the preceding night, only Mr. X is allowed access to the prisoner. This time is used for counseling, prayer, and whatever else Mr. X deems necessary. With this understood, Wanda takes Mr. X to death row for his first appointment. There, waiting for him, is MELNICK—the serial rapist and murderer also known as “The Ladykiller.”

What Melnick soon learns is that Mr. X is a beautiful woman—a gorgeous blonde who dispenses her own form of justice the night before each execution. An experienced seductress, she brings Melnick to the brink of sexual fulfillment only to finally knee him in the groin. The next morning, she puts him in the electric chair and disposes of him forever.

Her second appointment is with DECKER, a killer and cannibal who is kept chained to the wall of his cell. While Mr. X prepares for her night with him, Wanda spies the woman changing. She recognizes the blonde’s face, but can’t place it. Mr. X goes to work on Decker while Wanda wonders, “Where have I seen her before?”

The following day, Mr. X prepares for her third and final appointment with a man named RAY SWANSON, on death row for killing an off-duty police officer with his bare hands. Meanwhile, Wanda has been at the library, looking through old newspapers. Finally she discovers what she’s been looking for—a picture of the woman known as Mr. X. Her real name is ALEXANDRA NELSON, and she is Ray Swanson’s fiancée. She’s here to break Swanson out of prison!

Back at death row, Wanda confronts Mr. X at gunpoint. Handcuffed to a chair, Mr. X convinces Wanda of Ray’s innocence. There’s a safe in the warden’s office containing $300,000—Mr. X’s pay for executing the criminals. She promises to give Wanda half of it if she helps her and Ray escape. Wanda agrees, and the two of them hatch a plan that will put both Cheney and Hank in their place, give Ray his freedom, allow Alexandra to have her fiancé back, and let Wanda have a chance to start over in life.