Mrs. Frisby and the Rats of NIMH

Racso and the Rats of NIMH is a direct sequel to the Newbery Medal-winning book Mrs. Frisby and the Rats of NIMH by Jane Leslie Conly's late father, Robert C. O'Brien. Conly wrote her sequel long after O'Brien's death in 1973, so even though Conly's book attempts to answer many of the open-ended questions posed by the original, it is still Conly's work and not O'Brien's. One of the most anticipated events of the first book was the possibility of Mrs. Frisby's children (Teresa, Martin, Timothy, and Cynthia) having grown up to visit the rats themselves and partake in adventures of their own. Within this book, however, only Timothy is shown as having actually left for Thorn Valley. Teresa and Cynthia are still at home, and Martin has started a family of his own. While it ties up this plot point cleanly, many readers criticized this, feeling that the other children had much potential and deserved more exploration. Another question from the first book that Conly chose to answer relates to the fate of Justin, one of the rats who was a major character in the first book. In the end of the original story, O'Brien hints that Justin sacrificed his life when he went back to rescue a fallen rat, but Conly chose to keep Justin alive and well in her sequel.

Another subject in which more information was brought to light was that of Jenner. In the first book it was hinted that Jenner had died, but in an event prior to those of the book. Jenner had disagreed with Nicodemus over leaving the city, and took a group of separatists to start a new colony. In the end of the first book, it is revealed that NIMH had found a number of unusual rats dead, and it is assumed that this is Jenner's group.

However, according to the sequel, Jenner had in fact managed to escape the fate of the others (the first book having left it inconclusive whether "six or seven" dead rats were found, Jenner's group having had seven). Although he and the few surviving separatists had strong philosophical opposition toward the Thorn Valley group, they kept their distance, still believing that their way of life was better. In the sequel, it is revealed that Jenner had started a family, which Racso happens to be a part of. Jenner later makes a surprise appearance in the story and shows that despite his questionable loyalties, he cares strongly for his son. Some readers feel that the inclusion of Jenner as an antihero was heavily influenced by the film adaptation of the first book, The Secret of NIMH, in which Jenner played a much more central role than he did in the book. Jenner only turns up rarely in the book, mainly in flashbacks, and is killed off the page during the climax.

The Secret of NIMH

Despite a few plot similarities Racso and the Rats of NIMH has no connection to The Secret of NIMH's sequel, Timmy to the Rescue.