It is possible that after the book Mrs. Frisby and her family will journey out to see their friends, the rats of NIMH, in Thorn Valley. When it is time to move back to their winter home in the garden, the cinder block will be there ready and waiting for them. Timothy will most likely get sick again but probably not with pneumonia. Martin will probably do what he said in the epilogue, by putting something shiny out for Jeremy. Then, the family will be able to fly to the rats home on his back.