The comic strip was first published in the Mortadelo magazine, issues #112 (January 15, 1972) to #122 (March 26, 1973).
Professor Bacterio has invented a machine which is supposed to bring literary figures to life.
He attempted to test this invention with a copy of the fairy tale Snow White; but somehow the machine has malfunctioned and instead unleashed a group of horror monsters onto the city: Frankenstein's Creature, Dracula, a Werewolf, a Living Mummy, King Kong, a hideous Alien, a Witch, a Ghost, the Hairy Beast, and the most terrifying of them: The Thing (of which no deliberate depiction is shown).
Mortadelo and Filemon are called in to apprehend the fugitive monsters, who are one by one surprisingly revealed to be merely masked criminals assigned by their mastermind Macario Cabezón to rob gainful objectives all across the city, using their masquerade to terrify the populace into flight.
But the story ends with a twist when Mortadelo wishes to prove to Bacterio that his machine does not work by inserting a book about medieval monsters - and bringing a dragon to life, which chases Mortadelo, Filemon, Senor Super and Bacterio all over the country with its fiery breath!