Secret of nihm is my favorite Don Bluth movie. While a number of his films cater exclusively to the youngest of kids, which I suppose cartoons probably should, secret of nihm is at times definitely not for children.
Secret of NIHM follows the adventures of Mrs. Brisby, the recently widowed mouse taking care of her sick child. Because her late husband, Jonathan, died a hero, Mrs Brisby is destined to save the field from an unknown danger. While many of the creatures in the field, the land surrounding a farmer's house, lead simple lives, the rats are far from ordinary rodents. Living within the farmer's rose bush, the rats have mastered electricity and even some form of magic. All thanks to NIHM.
The National Institute of Mental Health has a vested interest in the rats, calling throughout the movie and asking about their abilities. Abilities the received after they were captured on the streets and injected with untested chemical formulas. A number of rats and mice we subjected to the trials and found they were learning to read and figure their way out of their cages.
They escaped and while most of the mice were lost in an air duct (a character points out they absolutely died, just one of the dark tones of the movie), two survived, one being Jonathan Brisby, who earned the rats' eternal gratitude by opening the final hate to their freedom.
NIHM wants to clear out the rats for whatever reason and they know it, preparing for a move to a safer place. Before they can go, they pledge to help move Brisby's home from the party of the farmer's plow. She can't move her family because her son is sick with pneumonia, being treated by the other surviving mouse from NIHM, Mr. Ages, wise intelligence manifested in medical knowledge.
Secret of NIHM is an incredible movie, early one of the best to come out of the 80s. And like many others of this age, it had it's fair share of villains. Though NIHM is treated as the big looking evil threatening all of the rats, there's a traitor on their midst with Jenner. He wants to take control of leadership by killing Nicodemus, the eldest and wisest among them. He uses Brisby's relocation as the best opportunity, cutting the ropes holding the cinder block home aloft and crushing the old rat under it.