A great adventure story, beautifully animated, funny, and a female protagonist to boot! Mrs. Brisby has to find courage to save her sick son, including going to the *scary* Great Owl for advice, and asking the Rats of NIMH to move her cinderblock house out of the way of the tractor-plow.
Two of the rats are bad guys and they plot a murder and see it through! The two of them also wind up killed, one with a sword and the other with a dagger.
Also, the flash-back story about how the Rats of NIMG escaped has some scary bits when they are sort of tortured, and the narrator sadly explains how some smaller mice who tried to excape in the air vents were blown away by the fans. Mrs. Brisby also finds out the true story behind the death of her children's father.
Now as a kid, I was very sensitive about death, but for some reason, this movie didn't bother me, I think because when bad things happen, characters show how to deal with it maturely. For instance, when Mrs. Brisby finds out how her husband died, she is sad about it but greatful to have been told, and understanding of why she couldn't be told before.
As with other better known classics by Don Bluth (All Dogs Go to Heaven, The Land Before Time, An American Tail), the adventure is captivating because of the real danger characters face, but the skill of storytelling, the tenderness in the voice-acting, and the lightheartedness of the music, all combine to make the adventures fun and not too scary.