[Opening lines]

In the beginning, they were just ordinary street rats. Until, they were captured by man and taken to NIMH. The National Institute for Mental Health. They went through experiments. For some strange reason, they become intelligent. They found, they could read. And so they escaped through the ventilation system. None of them would've survived, were it not for that incredible bravery of that great mouse Johnathan Brisby. But alas, he could not save himself. The wisest of creatures, the great owl once said, Johnathan Brisby was the bravest of them all.

Tonight we journey to Thorn Valley, We must leave no evidence that the Rats of NIMH ever existed.

But Johnathan Brisby's widow and her family stay behind in a home they'd always known. The prophet Nicodemus predicted that NIMH will again thrust its evil on the grass and a son of Johnathan Brisby will be chosen to save them.