Paco (AKA as 'Cocosabio'), his girlfriend Cayetana and Venancio 'el Torralba' commit petty thefts and, from time to time, spend a few days in jail, which is why "they play the piano" (in slang, "the police have a file on them"). But one day they meet don Federico 'el Tizona', a petty thief like them, but with the pretense of a man of the world. So don Federico convinces the gang that, using European methods, their success is guaranteed. Paco then decides to act scientifically to get hold of the surgical material from a hospital.