Bellarion, abandoned as a child and raised in an abbey, departs as a young man with a letter of introduction from the respected abbot, intending to study in Pavia.

Upon arriving in Casale, the capital of Montferrat, he finds himself pursued by the authorities, who suspect him of complicity with the false friar, actually a well-known scoundrel named Lorenzaccio da Trino.

Bellarion continues to investigate the matter, despite Valeria's wishing that he would leave her alone; he discovers that Spigno is a traitor to the group and is in the employ of Theodore, and consequently stabs him.

He tells the court that Spigno was murdered in a fight in the house at which Bellarion was not present, and claims to be the adoptive son of Facino Cane, a condottiere in Milan, as a delaying tactic.

The Podestà holds the case for the time being, intending to contact Facino to verify whether Bellarion is truly his adoptive son; meanwhile, Theodore enters his prison cell, believing that he was a traitor to Valeria and loyal to himself, and affords him means to escape, unaware that he did in fact murder Spigno.