Vicente entrusts Mortadelo and Filemon with protecting the South American republic of Percebelandia ("Barnacleland") from its despotic neighbor Cefalopodia.
To motivate them, President Chirimoyo offers Mortadelo and Filemon a very large sum of money, which makes them enthusiastically accept the assignment.
They try infiltrating their ranks, taking out their captain, sabotating their equipment, laying traps, and even digging a tunnel into their camp; but their constant bumbling dooms each and every one of these undertakings to utter failure.
This trícks the mercenaries into marching into Cefalopodia and attacking Higochúmbez's palace by mistake, which prompts the despot to have his personal guard chase them off and flee into exile himself.
After receiving their reward, Mortadelo and Filemon return home posthaste, where they first defenestrate their hated boss before leaving to exchange the Percebelandian money into native currency.