C. S. Lewis introduces The Voyage of the Dawn Treader with the words "There was a boy called Eustace Clarence Scrubb, and he almost deserved it." Eustace is the cousin of the Pevensie children, but he is nothing at all like them. In fact, Edmund and Lucy are not at all pleased when they must go to stay with Eustace and his parents over a school vacation.
Eustace, the narrator explains, has "read none of the right books." He focuses on books of information and never reads fairy tales of any kind. That is why, when Lucy, Edmund, and Eustace fall through the painting and end up with Prince Caspian on the Dawn Treader, Eustace has no idea what has happened or what to expect. And he is not happy about any of it.
Reepicheep the mouse is a special source of horror to Eustace. Reepicheep, of course, is no ordinary mouse. He is one of the Talking Beasts of Narnia. Reepicheep is about two feet high, walks on his hind legs, wears a crimson feather and a long sword, and, of course, talks. Reepicheep greets Lucy and Edmund with great courtesy, but Eustace exclaims in disgust, "Ugh, take it away." He hates mice, he says, and he loathes performing animals. "They're silly and vulgar and—and sentimental."
Again, because Eustace has not read the right books, he cannot accept Reepicheep for who and what he is. Reepicheep is, naturally, offended, and a rivalry soon springs up between Eustace and the mouse that Lucy, Edmund, and Caspian must work hard to keep under control.