In San Francisco, Peter Lachlan is driving down the street when his hand suddenly seems to act on his own, accelerating him forward into traffic. His foot clamps down on the accelerator on its own, and Peter accelerates the car toward a man crossing the street. He runs the man down and sees hallucinations of himself diving onto a tarantula. When Peter looks up, a crowd has gathered around him... including the man he thought he ran down.
Peter goes to see Dr. Ravadon and asks him to cut off his right hand. Ravadon examines the right and then the left hand and says that they appear normal, but Peter insists that if Ravadon won't do it, he'll find someone else who can. Ravadon asks for an explanation and it grabs a piece of paper and starts writing, seemingly on its own. Peter then shows Ravadon the note, who says that it's in Latin. The young man insists the hand belongs to someone else, but he doesn't know who. However, Peter warns that the hand wants him to commit murder, and has attempted three times. However, he warns that although he has stopped it each time, he knows that he'll fail the next time it tries. Ravadon refers him to a psychiatrist, but Peter says that he doesn't want one. When Ravadon says it's impossible, Peter grabs a bust and smashes it into his right hand.
Ravadon has no choice but to remove the damaged hand. Afterward, Peter insists that he had no choice and explains his company sent him to San Francisco to manage the new operation. He met with a new client, Franklin Heller, but was distracted going into the apartment and pressed the wrong buzzer button. He ended up at the door of Susan Douglas, and was engaged to her in three days' time. Peter explains that he sent Susan away to keep the hand from killing her, and insists that the surgery saved her life. The first time he realized something was wrong was when his hand dialed a number on its own. The hand then wrote down the name of the man on the other name, and Peter said "Borgus Weems speaking." After hanging up, the hand burned the note.
The doctor suggests that Peter tell his story to a psychiatrist, and Peter agrees. He explains to the psychiatrist, Dr. Innokenti, that the man he called, Brock Ramsey, came to his apartment. Ramsey said that he knew someone who lived in that apartment. As Ramsey talked, Peter's hand grabbed a letter opener and tried to stab the man. Peter manages to restrain it and tells Ramsey to get out. As the man flees, the phone rings and Peter answers it, and Susan asks to meet him.
Peter describes how days later, his hand took control again while he was driving down the street. His foot began to cooperate with the hand and tried to run down the bearded man. The next morning, Peter read an article about his car went out of control and almost killed Everett Winter, an attorney at law. After that, he began planning to kill Susan.
The right hand took Peter to illegally buy a gun. He then went to see Susan, unaware of what his hand had planned. He drew the gun on Susan and tried to stop his right hand, warning that it's becoming too strong. Peter tries to stop himself by committing suicide, and Susan stops him. He drops the gun and tells her to go far away so he can't kill her. It was then that he got the idea to have it cut off and went to see Ravadon.
Dr. Innokenti dismisses Peter as insane and leaves, and Ravadon suggests that Peter talk to a policeman friend. The officer, Nico Kazanzakis, recognizes the name Bogus Weems and explains that he lived in Peter's apartment. Weems was involved with the supernatural, and was pushed out the window to his death four years ago. Someone had cut off Weems' right hand at the wrist before shoving him out, but the killer was never found. Weems' niece and her boyfriend were suspects... and the niece was Susan Douglas. The lawyer who got her off was Everett Winterreich, and Peter believes that Ramsey was Susan's boyfriend, who was never identified. Peter figures that Weems' hand is trying to get revenge on the people responsible for his death, and Ravadon writes out a prescription for a tranquilizer. He realizes that the prescription is the same phrase that Peter wrote earlier: a line from Virgil, "Arise my avenger, out of my bones." Ravadon stares in horror at his possessed hand and begins to have visions of murder.