It's better to be hated than to be trapped forever in an immortal, immobile body.
It's a very didactic book. There's a mouse named Alexander, and people don't like him because he's a mouse and he steals people's food. Then he finds a wind-up toy mouse who is also intelligent, and the toy mouse is a little girl's favorite toy but can't move unless he's wound up. (Who knows how he manages to speak.) Alexander befriends the toy mouse, and they have conversations, and Alexander feels jealous because the toy mouse is well-loved, and the toy mouse tells him about a magic lizard who can change animals into other animals. So Alexander goes to the lizard and asks to be changed into a wind-up mouse, but the lizard tells him to come back with a purple pebble. So Alexander looks for a long time for a purple pebble, and the little girl has a birthday party where they got rid of all her old toys (which actually sounds like a good idea, as a parent) and the wind-up mouse is in a box, going to be thrown away, along with a purple pebble. So Alexander uses the pebble to turn the wind-up mouse into a real mouse. The end.
It's got a very clear message of "the grass is always greener on the other side". Leo Lionni's illustrations made of collages and patterns are really nice to look at, but the story isn't really very interesting or better than books like The Velveteen Rabbit. The message is very heavy-handed.