As indicated by the title, the main character of Mrs. Frisby and the Rats of NIMH is Mrs. Frisby, a small mouse living with her four children in a farmer's field. Each year, they leave for the forest or meadow and hide out until plowing is completed, and then return once the harvest have been laid in.

Although she was a widow (her husband had died only the preceding winter), Mrs. Frisby was able, through luck and hard work, to keep her family -- there were four children -- happy and well fed.
(O'Brien, Mrs. Frisby and the Rats of NIMH, Amazon.com)

When the book starts, her son Timothy becomes very ill and it becomes dangerous to move him. With little time remaining before the plowing starts, she goes on a dangerous quest to get medicine from the rats who live nearby. These rats have been mentally augmented by a local laboratory, and are smarter than the average animal; Mrs. Frisby's own instinctual survival sense and her general willingness to sacrifice give her the ability to survive and win over larger animals.