The Signoria puts 2000 men to work on the river project. When three workmen are discovered mutilated on the riverbank and a serpent with glowing eyes has been sited Leonardo is recalled from Milan to solve the mystery.

With the help of Niccolo da Pavia, Leonardo uses a device of his own to "snare the monster" but this disturbs another serpent of less imposing size but no less lethal. To trap Leonardo Niccolo's wife, Donna Elenora is abducted by the Viper and held in a castle. A plan to rescue her includes the commedia dell'arte troupe I Comici Buffoni.