Before it was a movie, The Secret of NIMH was a book: Mrs. Frisby and the Rats of NIMH. Published in 1971, Robert C. O’Brien’s book won the Newbery Medal in 1972.
Mrs. Frisby is a widowed field mouse. Her late husband was a companion of the mysterious and intelligent rats of NIMH.
NIMH is the National Institute of Mental Health. The rats had been subject to various experiments that left them highly intelligent and organized — more like humans than rats. They escaped the lab and set up a literate, mechanized society under a rose bush near where the Frisbys live.
Each year, the Frisbys trek to their summer home to avoid field plowing. An early thaw means it is time to head for the summer home, except for one problem — young Timothy Frisby is too sick to be moved. Mrs. Frisby encounters a crow, a murderous cat, and a wise old owl before eventually finding the rats of NIMH. They agree to help her because of their association with her husband. The rats engineer a way to move the entire Frisby home to a safe location!
O’Brien’s daughter wrote two sequels to Mrs. Frisby and the Rats of NIMH. Rasco and the Rats of NIMH is the story of a city rat who comes to join the colony of intelligent lab rats. R-T, Margaret, and the Rats of NIMH is the story of two lost humans who are rescued by the rats and later return to save the entire colony.
The book also inspired a 1982 movie called The Secret of NIMH. Many reprintings of the original book have the movie title, rather than the original title. The movie version also changed the main character’s name from Frisby to Brisby in order to avoid any legal issues with the Frisbee flying disc.