FAQ

Because it was prophesied that Timmy would save Thorn Valley from the threat of NIMH.

The book is Rasco and the Rats of NIMH. followed by R-T, Margaret, and the Rats of NIMH. Written by Jane Leslie Conly

However the movie doesn't follow up the book with the story of Timmy and Rasco sabotaging the project of building a dam.

Instead the movie introduces new characters Jenny the female mouse and Cecil the caterpillar trying to stop Timmy's mentally insane brother Martin in the city trying to conquer Thorn Valley.