Mrs. Brisby (Frisby in the original book) is a very shy, timid, and lonely mouse widow whose family is threatened by a premature warm spell, prompting the farmer to start his mowing early, endangering the cinder block she and her family call home. Normally she would simply pack up their things and move to a new spot in the field, but her younger son Timothy is too ill to be moved, forcing her to seek another alternative before it's too late. With the push of her local confidante, the Shrew, and the aid of a clumsy, love lorn crow named Jeremy, she goes to see the Great Owl, who, upon learning that she is the widow of a mouse named Jonathan Brisby, advises her to seek the aid of the mysterious rats who live in the rosebush outside the farmer's house. There, Mrs. Brisby learns of her deceased husband's past, and the secret of these rats, the Rats of NIMH, who agree to move her home before they abandon their underground colony for Thorn Valley, but Mrs. Brisby herself is endangered by the evil machinations of Jenner, a power hungry rat who seeks to overthrow the rats' leader Nicodemus so as to keep the rats in their underground colony, and will go so far as to endanger Mrs. Brisby's children to achieve his goals. Only Justin, the rats' gallant, Errol Flynn like captain of the guard, stands between Jenner and Mrs. Brisby, who must call upon the same courage her husband had if she is to save her family.