Mrs. Brisby, a widowed mouse, lives in a cinder block with her children on the Fitzgibbon farm. She is preparing to move her family out of the field they live in as plowing time approaches, however her son Timothy has fallen ill, and moving him could prove fatal. Mrs. Brisby visits The Great Owl, a wise creature who advises her to visit a mysterious group of rats who live beneath a rose bush on the farm. Upon visiting the rats, Brisby meets Nicodemus, the wise and mystical leader of the rats, and Justin, a friendly rat who immediately becomes attached to Mrs. Brisby. While there, she learns that her late husband, Mr. Jonathon Brisby, along with the rats, was a part of a series of experiments at a place known only as N.I.M.H. (revealed earlier in the story as the National Institute of Mental Health).