The chapter begins with the GHW explaining to the other witches that she will now demonstrate how her formula works.
The day before, she had given the Mouse-Maker (in the form of chocolate) to a greedy boy in the hotel, and set the alarm clock to go off today at 3:30pm, five minutes from now. Then she told the boy to meet her in The Ballroom at 3:25pm, where she would give him even more chocolate.
After some bustling of shoes, wigs, and gloves, in marches the boy: Bruno Jenkins.
Our narrator recognizes him. He's seen him around the hotel, always eating and bragging about his rich dad. The two boys even had a little run-in as Bruno was trying to kill ants with a magnifying glass and the narrator heroically stopped him. Basically, the narrator isn't too concerned about Bruno being turned into a mouse.
The GHW begins to count down. Bruno becomes more and more confused, and suddenly, he begins to transform, piece by piece... and just like that, he's a tiny mouse!
Cue the applause from the witches. Bruno, in mouse form, runs off.