Erotomania is the term used to describe a disorder where people delude themselves into believing someone is in love with them. Trying to use facts to convince them that this isn't the case usually won't stop these delusions, though it is possible someone with erotomania could become dangerous and violent when their obsession is challenged. And, sadly, most law enforcement and legal authorities don't take this very seriously...especially long ago. I mention this because clearly this story by Robert Bloch (the author of "Psycho" and many episodes of Hitchcock's TV shows) is writing about the disorder.
The story begins with David arranging his usual weekly purchase of roses...which at first seems like a nice gesture. However, you soon learn that he is sending them to a woman he dated long ago, Annabel (Susan Oliver). The problem is that Annabel is now married and has no interest in David. But this doesn't matter.... David insists that eventually she'll come to her senses and reciprocate. Her husband, not surprisingly, is angry and eventually he goes to confront him...and that's when you realize it's all part of David's plan...and David is like the spider and the husband is just a fly.
This episode is very much like "Psycho" but possibly even more shocking. At the big climax late in the show, there is a scene where, frankly, I am shocked they got it past the censors and sponsors. Unbelievably creepy, sick and, sadly, not nearly as far-fetched as it seems. One of those haunting episodes you won't soon forget....and you might want to think about this before watching as the episode might leave you shaken.
By the way, while I am no expert on erotomania, I am a retired psychotherapist and worked with many truly sick people....people not really that different from David. This is actually why I eventually changed lines of work!