In the late 1970s, a group of gifted and disgruntled animation artists left Walt Disney Studios and started their own company. Led by Don Bluth, they created “The Secret of NIMH,” their first animated feature film, based on a book by Robert O’Brien. A group of rats have escaped from the National Institute for Mental Health (NIMH) and created a home for themselves on a human’s farm. Because of the experiments they were subjected to, the rats have enhanced intelligence and siphon power from the farm to live better. Now they’re ready to move and live on their own, right after they help Mrs. Brisby move her house and children before plowing season starts. But there are rats that don’t want to move and plot to ruin everything. “The Secret of NIMH” has the look and feel of a classic Disney animated film but it’s a little bit darker in content. In many ways, it’s a lot more satisfying because of it.