Shmoop has a bone to pick with this ending: sure, the Frisbys end up safe in their vacation home, and the rats escape from the NIMHers once again, proving that brains beat brawn once again. But the thing that really kills us is that we don't know if the rats ever make it to Thorn Valley. If they do make it, we don't know how they fare once they get there. We are dying to know! The way we see it, there are a couple possible reasons for this ending:
We're going to go with option B—that the ending is left up to the reader's interpretation and that's why it is purposefully vague. The author leaves us with this image of the cuddly, snoozing Frisbys: "Outside, the brook swam quietly through the woods, and up above them the warm wind blew through the newly opened leaves of the big oak tree" (28.43). Certainly this is a happy ending for the Frisbys. But… no mention of the rats.
In a way, though, that makes perfect sense. After all, this book is all about the power of thinking for yourself and making your own decisions. In that spirit, what do you think about the ending? Are the rats living the life in the country? Or did something terrible befall them?