As the book starts, Mrs. Frisby and her four children are preparing for Moving Day, when they move from their winter home in the field to a summer home by the brook. However, her son Timothy contracts pneumonia, and moving him becomes hazardous:
He was supposed to stay in bed, and moving meant a long walk across the field of winter wheat, up and down the hill to the brook's edge, where the Frisbys made their summer home... the home itself would be damp and chilly for the first few weeks...
(O'Brien, Mrs. Frisby and the Rats of NIMH, Amazon.com)
Added to this danger is the possibility that plowing will begin early, putting their lives in danger. Mrs. Frisby seeks help from other animals, including the intelligent Rats of NIMH, and is in constant danger from predators, who see her as a meal. Her major problem is finding a way to cure or move Timothy before plowing starts, and then later finding a way to warn the Rats that their own home in the rosebush is in danger.