Mrs Frisby and her family have a problem: their nest is about to be ploughed over by the farmer. In return for helping a crow to escape the terrible farm cat he introduces her to the owl -- a frightening experience for a mouse. The owl's advice is to ask the rats for help. But when she visits their nest she discovers that the rats of NIMH are no ordinary rats, and Mrs Frisby finds herself being drawn into an increasingly exciting and fantastic enterprise.