Our narrator and Bruno, both still in mouse form run through the hotel and up the stairs toward Grandmamma's room.
They get there easily enough, but right outside Grandmamma's door, our narrator sees the maid who had ratted him out before for playing with his mice (no pun intended). He and Bruno hide in a pair of shoes outside the door.
Unfortunately, the maid decides to put her hand in the shoes, and when our narrator bites her (oops, he couldn't help himself!), she screams bloody murder.
Luckily, this causes Grandmamma to open the door, and our narrator and Bruno shuffle into her room.
The narrator shouts to his Grandmamma to close the door, and she does, despite the fact that she's frozen with fear. She knows exactly what has happened.
She cries and cries, while our narrator-mouse tries to comfort her. It's okay, he says – he's kind of excited to be a mouse. As long as she'll still take care of him, he's not too worried about it. Man, he really has a good attitude about all this.
While Bruno munches on some food in the room, our narrator tells his grandma the whole story. If you skipped Chapters 5 through 12, this is basically a quick summary.
He then asks his grandma how it is that he can still talk. She assures him that the witches have only changed his appearance, not who he is at his core. He's still her grandson, as he always was, he just looks a little different. He's a "mouse-person" (14.53).
Then, our mouse-person has an idea. They can't let the witches get away with their plan, and he wants to stop them.
He remembers that the GHW said her room was number 454, and he's pretty sure he is staying in room 554. If her room is directly below his, he could climb down the balcony and into her room. Since he knows she's at a tea now, he wouldn't get caught. Then he could steal a bottle of the Mouse-Maker and they could – wait for it – turn all the witches into mice. Who's the Brilliant One, now?
Our narrator and his grandma are both extremely excited about the plan, and after checking to make sure that the narrator's room really is right above the GHW's, they decide they will lower our beloved mouse-person onto the balcony below in a half-finished sock that Grandmamma was knitting. Thank goodness for grandma hobbies.