In the Simpson Case, Richard Diamond, a private detective, is approached by Mrs. Simpson, who is deeply concerned for her husband, John Simpson. Recently, John has been the target of an assassination attempt while tending to his garden. Although he survived the attack unscathed, the incident has left Mrs. Simpson anxious and determined to secure her husband's safety. She insists on hiring Diamond, despite John's objections to involving a detective in what he believes is a personal matter.