Chasing a petty criminal named Banco through the sewers of Rome, Signor Renzi, Commissioner of Police, hears a scream from the criminal. When Renzi and Pietro, the officer accompanying him, find Banco, the criminal is missing most of his flesh. They get lost in the sewers, and soon find themselves running from the Green Horde, a group of tall, cannibalistic, humanoid, underground dwellers. Sacrificing Pietro's life to save his own, Renzi finds the monsters wish to cut a deal with the commissioner of police. Renzi is to provide the Green Horde with people to eat, while they provide him with a good deal of ill-gotten gains.