R-T, Margaret, and the Rats of NIMH, like the preceding volume, Racso and the Rats of NIMH, was written by the daughter of Robert Leslie Carroll Conly (pen name Robert C. O'Brien), the author of the original Rats of NIMH book.

When two children, Margaret and her younger, autistic^[1] brother Artie ("R-T"), are lost during a camping trip, they are found by the colony of rats, specifically one named Christopher. The children help the rat community with various tasks, and Artie and Christopher become very close friends. However, when winter comes, the rats cannot shelter the children and must send them back. The children try to keep the secret of Thorn Valley, but after pressure, Margaret ends up revealing it. The story ends with a party of adults traveling to Thorn Valley to discover the rats' colony only to find an empty, apparently uninhabited plot of land with all traces of the colony removed.

The whereabouts and fate of the rats of NIMH are left unstated, though Artie does find a gift from Christopher, a picture of an arrow, presumably pointing to the new location of the colony.

• R-T, Margaret, and the Rats of NIMH title listing at the Internet Speculative Fiction Database