Mrs. Frisby, a widowed field mouse, must save her family when their home is about to be destroyed by a farmer’s plow. Her son is very sick, which makes moving their home extremely difficult.
While seeking help, she discovers a secret group of highly intelligent rats living under a rosebush. These rats were once laboratory animals at NIMH (National Institute of Mental Health), where experiments greatly increased their intelligence. They escaped and built an advanced underground society.
The rats agree to help Mrs. Frisby move her home to safety. In return, she learns about their past and their hidden existence.
In the end, Mrs. Frisby’s family is saved, and the rats are forced to leave their home to escape humans, starting a new journey elsewhere.