Barnaby Jones was a retired private investigator, but quickly came out of retirement after his son Hal, who had taken over Barnaby's job, was murdered. Hal's widow, Betty, helped Barnaby try and find her husband's murderer. The murder was solved, but Barnaby decided he wanted to return to investigating full-time and Betty decided to join him.
Betty is suspected of killing her two cousins so that she may grab the family's fortune all for herself.
First, cousin Donald dies and Betty decides to investigate the death. But as she goes about it, she is being kept under surveillance by a private investigator named William Torkington, employed by an Insurance company.
Torkington comes up with reasons to believe that Betty killed Donald and that she will continue to kill other relatives who might stand in the way of a family inheritance.