This is based on one of the best of the original Perry Mason books, from 1942. The writers of the TV show changed a few things, but it is remarkably similar in basic plot.
A lot happens. Indeed, it's almost too much for the 50 minutes or so of a TV episode. The basic story is that ten years earlier wealthy Franklin Shore disappeared mysteriously. Just about everyone is convinced he is dead, but widow (?) Matilda won't have him declared dead and the will probated. This upsets Franklin's brother, Gerald, who could use the money he would inherit.
Then one night niece Helen gets a phone call -- from Uncle Franklin! He's not dead! He wants Helen to meet him but tell no one. However Helen does tell Gerald who gets Perry Mason to come along to the meeting place. They find a note from Franklin saying to go to a certain place and meet someone else, a Mr. Leech. They go there but find him dead.
Meanwhile, the family cat has been poisoned! Later that night Matilda reports she has been poisoned! The next day Helen and her boy friend are shot at by someone hiding in a dark bedroom. That is an amazing a mount of action - -almost too much.
The story is unusual in that there is never a court room scene, and no one is ever arrested. Perry unmasks the murderer at a group meeting where Hamilton Burger is also present, much like an old Agatha Christie story. Hamilton Burger is not an antagonist in this story at all.
This is a well done, well plotted mystery story. It differs from the original book in that in the book Hamilton Burger is not so friendly. He accuses Perry and Della of hiding a witness, and arrests Della. Della goes on trial!
Highly recommended -- but the book is better.