The word “perfidy” is defined as “being faithless or disloyal,” so it is easy to understand why this word comes to Despereaux’s mind as he is being led to the dungeon. Perfidy underlies this entire scene from The Tale of Despereaux. The little mouse is going to jail because his family has turned on him. They are more devoted to the “law” than to their youngest member.
Despereaux has violated the mouse community’s legal code by sitting at the foot of the human king and touching his daughter, Princess Pea. Despereaux is summoned before the Mouse Council to answer this charge, and it is his brother Furlough who exhibits perfidy by searching for him and who delivers him to his trial.
“Make way, make way!” shouted Furlough. “I have him right here. I have Despereaux Tilling, who has been called to sit with the Mouse Council.”
Furlough displays no regret or sadness when he turns his brother over to face trial. But Furlough is not the only family member who turns on Despereaux. His father, Lester, participates in the trial by beating the drum. His mother, Antoinette, who is trying to save him by convincing him to renounce his actions, eventually resorts to attacking him.
“Mon Dieu!” shouted his mother. “Son, do not act the fool. Renounce! Repent!”
Ironically, the mouse community sees Despereaux as the faithless one. He has chosen loving a human over the “sacred, never-to-be-broken rules of conduct for being a mouse.” By allowing the princess to touch him, he is turning his back on the mouse code.
“This trial is about you being a mouse,” shouted the Most Very Honored Head Mouse from high atop the bricks, “and not acting like one!!!”
Breaking ranks with his species is punishable by being separated from his own for the rest of his life. The dungeon is the home of the rats, who will turn on him and probably kill him—the ultimate consequence for his perfidy.