The main characters are:
Fenton Hardy (the father of Joe and Frank) is a famous detective who lived in New York and hunted down various crooks, ending their careers as crooks by calling the cops upon their heads and getting them slapped into jail, also encouraging his two sons to do the same.
Joe Hardy is the oldest son of Fenton Hardy and is interested in solving mysteries with his dad, foiling would-be scumbags, and occasionally riding around the countryside on his motorbike with his brother, Frank.
Frank Hardy is Joe’s brother and enjoys many of the things that his brother likes to do. He also has a motorbike, and might crash while he was on it -foreshadowing-.
Chet Morton is the Hardy Boys’ chubby friend, who changes his expensive hobbies at least once a book. Luckily, this time, he is obsessed with trying to soup up his jalopy (don’t ask me what that is, but I think it’s a car of sorts).
The villain in the book stole lots of jewels and gold from a rich man’s house. He was famous in New York for stealing an actor’s costumes and using them to hide from the law.