Buffalmacco and his friend Bruno Degli Olivieri make a joke of Calandrino, forcing him, for ten florins, to buy a stone that should make invisible the one who owns it.

A black-out occurs during the meeting of a gang of criminals. When the light is back one of them is found killed with a stab on his back and all the clues point to a penniless man who had an argument with the victim a short time before the murder.