The two klassik funnymen star as baggage clerks who stumble across some alarming luggage: the remains of Dracula and Frankenstein's monster. Soon, the fiends are reanimated, and the comedic duo are joined by a monster hunter... who turns out to be the wolfman. It's a veritable monster mash, which makes the obvious omissions of its title seem really odd.
The reason, I think, for this film's classic status is because, like The Comedy of Terrors, it has the atmosphere of a no-shit-for-real horror movie despite being a comedy. This is bolstered by the presence of Bela Lugosi and Lon Chaney, Jr. reprising their famous roles as the monsters.
If I'm being honest, however, I have to admit that it didn't make me laugh as much as I had hoped. Too many jokes repeat the same few gags: the monsters do something in the background while no one notices, Lou sees something fishy but Bud doesn't believe him, etc. The duo have their moments, but I don't think I'm being controversial when I say that Abbott and Costello are no Laurel and Hardy. The film is interesting and intermittently funny, but not the all time classic its reputation suggests.