has been informed that a staff member of Circus Sidral is using his job as a front for smuggling an unknown type of contraband across the border.
Mortadelo and Filemón are tasked by El Super to find the culprit and secure the contraband.
At first they are hired on as caretakers for the sick animals, but as the plot develops, the circus director also asks them to fill in for performers who have become victims of some accident... with predictable consequences: Each of these episodes ends with a circus staff member posting a notice that the corresponding act will no longer be performed.
When Mortadelo tries to open the crates with a hammer and chisel, their contents are revealed to be sensitive high explosives, whose detonation completely eradicate the circus.
Thence, to compensate for the damage they have caused, Mortadelo and Filemón are forced to abdicate their entire salary payments - up to the end of their lives - to the ex-director's benefit.