Florence is at war with Pisa. Leonardo is commissioned to divert the river cutting off Pisa from the sea and thereby protecting Florence. At this time the political and social climate in Florence is unfavorable to Leonardo and he is encouraged to accept the invitation of the French King to come to Milan.

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...