Despereaux saves Princess Pea from Roscuro's plot by going to the princess and Mig and then helping them convince Roscuro to let them go.
After Roscuro, a rat, frightened Pea's mother to death, Pea insulted him. He plotted with Mig, a servant girl, to chain Pea in the dungeon forever. When Despereaux, a mouse, gets wind of the plot, he plans to save Pea and warn the king. However, the king won't listen to him.
Despereaux decides to save Pea himself. After getting the thread from the Threadmaster and eating soup that the cook gives to him, he heads into the dungeons and is led to Pea and Mig. Mig has refused to chain Pea, but Roscuro refuses to let them leave the dungeons.
When he smells the soup on Despereaux's whiskers, though, he remembers the night he scared the Queen. Pea promises that Roscuro can eat some soup in the dining room once he leads them out. Everyone enjoys soup together, and Desperaux and Pea become good friends. Mig is reunited with her father, who was a prisoner in the dungeon.
Desperaux is able to bring together multiple characters—principally himself, a mouse; Roscuro, a rat; and Mig, Pea, and the Cook, who are humans, to convince Roscuro to let go of his revenge, save the princess, and help Mig reunite with her father.