Arnold has always disliked going on Ms. Frizzle’s fieldtrips, so when he discovers that he’s permitted to stay behind at the classroom during the next one, he becomes ecstatic. However, when Ms. Frizzle gets the magic school bus to shrink down so that they can go to their planned destination, Arnold inadvertently eats the class and they’re forced to navigate through Arnold’s body in order to try to get out before it’s time to go home.