Back in the room, after scolding Bruno for not piping up while he had the chance (his mouth was full, he claims), Grandmamma proclaims that it's time to come up with a plan for how to get all the witches to drink the Mouse-Maker.
Our clever narrator, somehow even more clever now as a mouse, suggests that he sneak into the kitchen and put the formula right into their food. He'll have to pay close attention to know which food is theirs, but he thinks he can do it.
They both know this is very dangerous (apparently, Grandmamma is okay with her grandson risking his life every few minutes), so they make sure that he's as prepared as possible. Our narrator practices holding the bottle and unscrewing the top – success on both counts.
They decide they'll head down together and that Bruno will tag along in Grandmamma's handbag.
Grandmamma gives her grandson a few too many warnings, which kind of freaks him out, but in the end, she praises him for his bravery.