After our narrator's parents die in a car accident, he goes to live with his grandma in Norway. He loves it there, mostly because he loves his grandma.
She tells him all sorts of stories about witches who spend their time squelching children. These stories excite him and scare him all at the same time.
He and his grandma move back to England where he grew up, and one day, he has a face-to-face encounter with a witch. He's smart about it, though, and comes away without a scratch.
While he's on vacation with his grandma at a big hotel, he accidentally finds himself locked in a room full of scary, ugly witches – and ugly is putting it nicely.
Hiding behind a big screen, he overhears the witches' plan to turn all of the children of England into mice. This can't be good.
Just when he thinks he's safe, the witches find him hiding, and turn him into a mouse. Just like that, our narrator has gone from human to rodent (but he can still talk).
He goes to his grandmother for help, and, together, they figure out a plan. As a mouse, he's able to sneak into the head witch's room and steal some of the formula that the witches were planning to use to turn children into mice.
Our narrator takes this formula into the hotel kitchen and manages to dump it into the witches' dinner, which eventually turns them all into mice. Chaos ensues.
Still (and forever) a mouse, our narrator heads back to Norway with his grandma. He learns that he only has a few more years to live, and decides he will use those years to take down the rest of the witches in the world.