Mrs. Frisby returns to her home with her precious package containing some medicines to treat Timothy' pneumonia. The other children, waiting for her, look scared and relate her the news of Timothy's health. She reassures them and administes the medication to her son. Three days without setting foot outside, Mrs. Frisby is at the bedside of her son and realizes the improvement of his health as predicted by Mr. Ages.
On the fourth day, she goes outside and notices with joy, that the spring seems to take place and the winter to disappear. But soon, she couldn't refrain to think about “the moving day”. Indeed, as soon as the weather will allow, any animal that lives in the garden should have to leave the garden in order to escape alive from Mr. Fitzgibbon tractor plowing.
The thought of an early "moving day", confirmed during a conversation with a lady shrew, saddened her because Timothy can’t make any trip in the current state of things, due to his disease, without a risk to fall extremely sick and to die. And as if things could only get worse, she hears Mr. Fitzgibbon starting his tractor.
Source : O'Brien, R.C (1971). Mrs. Frisby and the Rats of NIMH. New York: Aladin Paperbacks.