Continuity mistake: At one point during "Flying Dreams" when Timothy is being cared for by Mrs. Brisby and the other children gather at his bedside, his left arm appears in various positions as the shots change - instantly on top of and then beneath the quilt. (00:18:35)
Continuity mistake: When Nicodemus is telling Mrs. Brisby that he looked at the words under the latch on the door and understood them, there is a clear, large label under the door. Then a different angle is shown and there is no label on any of the doors, but you should be able to see it still. (00:52:25)
Continuity mistake: While Mrs. Fitzgibbons is in the bedroom talking to her husband about NIMH, she pulls a window shade all the way down. But from the next view, where several rats are transporting an extension cord across the front yard's walkway, the shade is back to its original position again - about halfway up. (00:19:55)
Answer: He doesn't per se, the rats release him when they show up to move Mrs. Brisby's house.
Grumpy Scot