An unknown culprit uses an unprecedented process to switch the characteristics of selected animals, trading their special abilities with one another, and then sending them to commit high-level robberies.
Examples of this include: While investigating these strange occurrences (and constantly attempting to evade paying the entrance fee to the zoo), Mortadelo and Filemon gradually discover that a strange old man appears responsible for these personality switches, which also works on humans (as Filemon discovers when the subject imprints him with a bull's ferocity).
At the end of the story, Mortadelo and Filemon attempt to ask the zoo director, Xim Pancé, for any clues, but in the process discover that he is the culprit.
Using a device of his own invention, Pancé instills the behaviour patterns of a cat and a dog in Mortadelo and Filemon, but is adversely affected by their subsequent scuffle and chase.
When the device is wrecked as a result, Mortadelo and Filemon regain their original personalities, but when Mortadelo, with Filemon in his arms, attempts to jump off the roof they have landed on, he forgets that they no longer have the abilities of cat and dog, and thence make a crashlanding onto the street.