Mrs. Frisby and the Rats of NIMH is a children’s science fiction novel written in 1971 by Robert C. O’Brien. It tells the story of a field mouse whose son becomes ill as moving day approaches, so she enlists the help of a group of highly intelligent experimental rats for help. Robert C. O’Brien was inspired to write the Rats of NIMH after a visit to the National Institute of Mental Health’s experimental rat compound in the 1960s run by John B. Calhoun, who hoped to create a rat utopia but found that overcrowding and boredom repeatedly led to civilization collapse. The novel received broad critical acclaim at the time of its publication and won the 1972 Newbery Medal, the Mark Twain award in 1973, and was nominated for several others. In 1982, it was adapted into an animated movie, entitled The Secret of NIMH. It was followed by two sequels written by O’Brien’s daughter Jane Leslie after his death.

This guide uses the 1986 Aladdin Paperback Edition of the novel.

Plot Summary

The story begins as Mrs. Frisby awakes one February morning to the smell of spring outside. She crawls out of her tunnel which leads to the cinder block home where she and her four children, Timothy, Cynthia, Teresa, and Martin, live. Mrs. Frisby knows that spring means the farm will be plowed soon, and she has to move her family out of the plow’s way before that happens. She finds that Timothy is sick with what appears to be a fever and cough and decides she must go see Mr. Ages, an old white mouse, who is particularly smart and who creates medicines using herbs, seeds, mushrooms, and other natural things. Mr. Ages gives her a tonic that will keep Timothy’s fever down and help him heal but warns that it will take at least a month for Timothy to heal completely. During that time, he cannot be exposed to cold air. This presents a new problem for Mrs. Frisby, as she knows they only have limited time before plowing day.

On her way home from Mr. Ages, Mrs. Frisby finds a crow named Jeremy stuck to a wire. She helps him get free, and in return, he offers his favors any time. Later on, Mr. Fitzgibbon the farmer starts the plow and announces that they will begin plowing in just five days. Mrs. Frisby is at a loss until she realizes she can ask Jeremy for a favor. He has no information other than knowing about a wise old owl who lives deep in the forest and may be able to help. Jeremy takes Mrs. Frisby to the owl, who advises her to seek out the rats that live inside the rosebush. Mrs. Frisby knows almost nothing about these rats other than they keep to themselves, seem to be planning something, and steal metal from the farm. Still, she is courageous and goes to see them, and when they deny her, Mr. Ages explains that she is the wife of Jonathan Frisby, at which point the rats become most welcoming to her.

Mrs. Frisby finds that the rats have built an entire underground world. There are at least a hundred of them, they are highly intelligent and much bigger than normal rats, and they seem to hold her late husband in high esteem. Mrs. Frisby notices that the rats also have electricity and running water. While the rats hold an important meeting, Mrs. Frisby is asked to wait in their library. She finds that they can read and write very well; she also finds they are planning something and that they come from a place called “NIMH.” Nicodemus, the leader of the rats, soon explains everything to Mrs. Frisby.

The rats originally lived as normal rats in a marketplace full of delicious foods. One day, they are captured by men in white and brought to a laboratory. There, they are separated, caged, and injected with DNA designed to make them more intelligent. The experiment is a success, and the rats slowly become more intelligent than any other animal before them. They are made to run a maze, which fools them into thinking they will eventually reach freedom, but they never do. Eventually, they become smart enough to figure out how to escape, all with the help of some mice who are being given the same injections. One of these mice is Jonathan Frisby, Mrs. Frisby’s late husband. Another is Mr. Ages.

The rats attempt to create a new life with their newfound intelligence. The experiment also inadvertently extended their lifespan, so they feel they have ample time to figure things out. They stay in an estate for a season before making their way to the Fitzgibbon farm. On the way, they encounter a toy tinker’s truck filled with tools, mechanical bits, and food. They use this to create their underground world at the farm. Over time, the rats become dissatisfied and listless, feeling they have trapped themselves within a lifelong rat race to obtain more and more luxuries but never actually progress. They are too comfortable and too lazy. As a result, the majority of the rats decide to develop a plan to leave the farm and create a new life in the valley beyond the mountain. Some of the rats, specifically Jenner (Nicodemus’s best friend) and a few others, disagree with the new plan and defect from the group.

In the present, the rats devise a plan to help Mrs. Frisby’s family stay safe on plowing day. They decide to move the cinder block to the sheltered side of a large stone, so the plow will not go over it. The plan requires Mrs. Frisby to put sleeping powder in the farm cat’s food to keep it from attacking them, and she succeeds in doing so, but is caught by the farm boy. Justin, a friendly and humored rat, comes to her rescue and saves her. The cinder block is moved and Mrs. Frisby’s family is safe.

Jenner and the group of defectors are found dead at a hardware store in a town nearby. They were attempting to steal a motor when it shocked them all. The government and NIMH both suspect they are the rats from the lab and go to investigate. When Mr. Fitzgibbon mentions he has rats at his farm, the inspectors come to the farm to gas the rosebush in which they live after plowing it down. This puts a rush on the rats’ plan to leave the farm, but they thankfully have already moved most of their stores to the new home and manage to escape. Two rats do not make it out before the chamber is gassed, but it is never revealed which two rats it was. The others make their way into the hills, and Mrs. Frisby and her family make their way to their summer home, healthy and safe.