Rome is on the thresh-hold of the Holy year 1925, the city wishes to present itself as 'spotless' to the outside world. The appearance of a child's corpse puts an end to these aspirations. The police arrest a small time crook who has committed a robbery but has nothing to do with the murder of the child. His 'fence' the jeweler BALETTA, who is also a member of the ruling political party, suggests a deal: the crook goes free and in return Barlett will spill the beans on a planned robbery of the treasury in the Vatican. The police agree to the deal, and soon after one of Barlett's contacts is found dead. The police send officer Guglielmo Motta, who is not known in Rome, under cover as a wealthy 'fence' ( dealer in stolen goods). He slips into the criminal milieu thanks to the helpful charms of the lovely Lulu- they fall in love. Guglielmo gets scared when the body of Barlett shows up, and Lulu treats him with a new coldness. He wants out. The police chief dismisses this request. To top all of this Guglielmo's wife is jealous, and his mother in law seriously ill. Lulu hasn't missed the fact that Guglielmo has gone on ' home leave' and disappointed, she is prepared to work with a mafioso who knows the 'real' Guglielmo. Back from holiday Guglielmo ends up making contact with the Vatican robbers through a criminal intermediary. He is supposed to fence the goods. After the Vatican robbery has gone off with out any hitches, he waits at the agreed location for the hand over. Thanks to a letter written by Lulu, who regrets her double dealing and has out of desperation committed suicide, Guglielmo knows about the mafia gangster. And indeed, he is able to arrest all of the robbers, and retrieve the goods. An unsung hero, Guglielmo returns to his quiet life at the side of his wife Maria.