The Secret of NIMH
In 1998, MGM released a direct-to-video sequel called The Secret of NIMH 2: Timmy to the Rescue. In the sequel, Timothy goes to a place called Thorn Valley because he wants to become a hero like his father. His brother Martin has turned evil (although he does not want to), and Timothy must fight him in order to save the Rats of NIMH. At Thorn Valley Timothy learns from a young girl mouse that the mice who were thought to have been killed during the escape from NIMH are still alive, so he and the rats try to rescue them.