It's Abbott and Costello and Universal monsters, I mean why not.
The film is about two friends who are railroad baggage clerks. They receive a strange shipment, that ends up being the remains of Dracula and Frankenstein's monster. These monsters, of course, aren't dead and they escape to a remote island. Abbott and Costello then follow them and run into a mad scientist who wants to switch Costello's brain with that of Frankenstein's monster.
The film does merge horror and comedy very well, but it's better not to think about this one being an actual Universal monster movie.
Some of Abbott and Costello's funniest bits are in this film and the great thing about this movie is its ability to still be funny years later.
I strongly recommended this film to not only horror fans but fans of film.