Jeff Ashton is sitting at home one day when a mail carrier delivers a strange package. Upon opening the wrapped item, Ashton is horrified to see that the package contains a severed human arm. He immediately contacts his friend, Doctor Ray Sanders, and the two discuss an incident five years before when, as part of a group of cave explorers, they were trapped by a cave-in. In order to avoid starvation, the group had cut off the arm of one of their fellow cavers, a man named Ted Rogers. However, moments after doing so, the entire group was rescued and they quickly concocted a cover story for the authorities, saying that Ted had lost his arm in the cave in and not as a result of the group cutting it off with a knife.
Jeff and Ray try to find Ted, enlisting the help of one of their former caver friends, Mark Richards, who is now a police detective. Mark finds Ted's daughter (Teddy, named after her father) but she initially refuses to help the group find her father who has now apparently disappeared. However, after Jeff's friend Herman, another one of the cavers who is now a radio personality, is brutally killed on the air of his radio show by having his arm hacked off, Teddy agrees to help the group find her father.
The killings continue until only Jeff and Mark remain. They attempt to trap the killer, who they believe is Ted, but Mark has his arm ripped off after being pushed from a cliff with a rope around his wrist. Jeff is then knocked unconscious by the killer after a struggle. When Jeff awakens, he is in a bare white room and it is revealed that Ted's daughter Teddy was behind the killings. Her plan was for her brother Roger, masquerading as their father (who is now in fact a catatonic) to kill Jeff's friends and then kidnap Jeff, as the original leader of the caving group who came up with the idea to cut off her father's arm. Jeff is then sealed in a room to starve to death, with the only way to avoid death being for Jeff to cut off his own arm. The film ends with Jeff, imprisoned alone, shouting "Never!"
