The Secret of NIMH - Plot

Plot

Mrs. Brisby (Elizabeth Hartman), a shy and timid field mouse, lives in a cinderblock with her children in a field on the Fitzgibbons' 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. She visits Mr. Ages (Arthur Malet), another mouse and old friend of her late husband, Jonathan, who diagnoses Timothy with pneumonia and provides her with some medicine from his laboratory. Mr. Ages warns her that Timothy cannot go outside for at least three weeks or he will die. On her way back home she encounters Jeremy (Dom DeLuise), a clumsy but compassionate crow. They both narrowly escape being eaten by the Fitzgibbons' cat Dragon.

The next day, Mrs. Brisby discovers to her horror that Farmer Fitzgibbons has started spring plowing early. Although Auntie Shrew (Hermione Baddeley) helps her disable his tractor, Mrs. Brisby knows she must come up with another plan. With the help of Jeremy, she visits the Great Owl (John Carradine), a wise creature living in the nearby woods, to ask for help. He tells her to visit a mysterious group of rats who live beneath a rose bush on the farm and ask for Nicodemus.

Mrs. Brisby enters the rose bush and makes her way down to the rats' home, where she is amazed to see their use of electricity and other human technology. She meets Nicodemus (Derek Jacobi), the wise and mystical leader of the rats, Justin (Peter Strauss), a kind and friendly rat who is the Captain of the Guards, and a ruthless, power-hungry rat named Jenner (Paul Shenar). From Nicodemus she learns that many years ago her late husband, along with the rats and Mr. Ages, were once part of a series of experiments at a place known as NIMH (which stands for the National Institute of Mental Health). The experiments had boosted their intelligence to human level, allowing them to easily escape. However, the rats have concocted "The Plan", which is to leave the farm and live without stealing electricity from humans. Nicodemus then gives Mrs. Brisby an amulet called 'The Stone', that gives magical power when its wearer is courageous.

Because of her husband's prior relationship with the rats, they agree to help Mrs. Brisby move her home out of the path of the plow. She volunteers to drug Dragon, so that they can complete the move safely. Only mice are small enough to fit through the hole leading into the house; Jonathan was killed by Dragon in a previous attempt, while Mr. Ages broke his leg in another. Later that night, she successfully puts the drug into the cat's food dish, but the Fitzgibbons' son Billy catches her and convinces his mother to let him keep her as a pet. While trapped in a birdcage, she overhears a telephone conversation between Farmer Fitzgibbons and NIMH and learns that NIMH intends to come to the farm to exterminate the rats the next day. She manages to escape from the cage and runs off to warn Justin.

Meanwhile, the rats are in the process of moving the Brisby home using a rope and pulley system during a thunderstorm. However, Jenner, who is strongly opposed to The Plan and wishes to remain in the rose bush, sabotages the ropes with his hesitant accomplice Sullivan (Aldo Ray), causing the cinder block to fall and crush Nicodemus, killing him and making it look like an accident. Mrs. Brisby arrives and tries to convince the rats that NIMH is coming and that they must leave immediately, but Jenner, angered by her claims, attacks her and attempts to take the amulet from her neck. Alerted to the situation by Sullivan, Justin rushes to Mrs. Brisby's aid; a sword duel between Justin and Jenner ensues, ending with a mortally wounded Sullivan killing Jenner and saving Justin's life.

Mrs. Brisby suddenly realizes that the house is sinking in the mud it landed in. Despite the best efforts of the rats, they are unable to raise it from the muck. However, Mrs. Brisby's will to save her children gives power to the amulet, which she uses to lift the house out of the mud and move it to safety from the plow. The next morning, the rats have already gone to Thorn Valley with Justin as their new leader and Timothy has begun to recover. Jeremy also finds "Miss Right", an equally clumsy crow, and the two fly away together.