Mrs. Brisby's children leave for school at Thorn Valley, Martin hopes to become a brave hero like his father while his brother Timothy falls in love with a girl rat named Jenny, who has a bit of a secret role. Thorn Valley is threatened when humans takee over and capture the rats. To help rescue Jenny and the rats, Mrs. Brisby comes to Timothy and Martin's aid, and a new adventure begins to the worst of all places, NIMH.
