A mouse is born and is not considered "normal" because he is so small with ears too big and his eyes open. His family is disappointed in him because he doesn't do mousely things. What he does do is read and reread a story about a princess and her knight. He hears music and discovers the king singing and playing to his daughter Princess Pea. Despereaux falls deeply in love with the princess and resolves to be her knight in shining armor. Desperaux is exiled to the dungeons and its deadly rats by the mouse council because he sat at the foot of the king and is endangering the mouse way of life. In the dungeon he discovers a plot to kidnap and eventually kill Princess Pea by a delusional young servant girl and a rat bent on living upstairs in the castle in the light denied him in the dungeon. Of course there is a happy ending with Despereaux restored to the mouse community and the princess with mousely friend living happily ever after.
The previous post leaves out most of Despereaux's quest to rescue Princess Pea from the dungeon! [Gasp!] Despereaux actually CHOOSES to go back to the dungeon to rescue his true love. Hoping to gain a mouse-meal, Roscuro the rat actually leads Despereaux directly to the princess. Once there, Roscuro has an epiphany (due to the smell of soup) that he never wanted to do all of these bad things, ... only to experience light and soup. Princess Pea offers Roscuro soup and he leads the group out of the dungeon to a new life of adventure and friendship. And although Despereaux, being a mouse, never gets to marry his true love (Pea), he does gain a confidant and an adventurer.