R-T (his real name is Arthur) and Margaret go on a camping trip and accidentally get lost in the woods. They are rescued by Christopher and Racso, a couple of escaped lab rats who live in a rat village in Thorn Valley. During their adventures, the rats and the human children learn how to cooperate and care for each other.
This is the third book in the Rats of NIMH series. Like the others, it is an enjoyable, often humorous story. I especially like this one because it shows a character, Margaret, who starts off as one type of person (not a very nice one) and becomes someone better. The best book is still the first one, Mrs. Frisby and the Rats of NIMH, but both this one and Racso and the Rats of NIMH are also good.