The nature of the rats's Plan - normally spelt with a capital P to emphasise its importance - is not fully revealed to Mrs Frisby until late in the book, when she has been taken fully into the rats' confidence. It is nothing less than the rats' attempt to form a genuine civilization of their own. Of course, it is only due to the fact that their mental and intellectual capacities have been so enhanced by the scientific experiments conducted upon them by humans that they can even conceive of anything so ambitious, but, as Mrs Frisby reflects:'It's a good plan, and a brave one.'
The Plan is not only ambitious, but wholly admirable, because it springs from the honest desire of most (although not all) of the rats to create a self-sufficient society in which they do not need to depend on anyone else at all. Up until that point, after escaping from the scientific laboratory at NIMH, they have been living in comfort, close to human settlements, with all the technological advances that other animals could never even dream of; they not only have enough food and shelter but the use of electricity, machinery and all manner of modern gadgets. However, all these things are stolen from human civilisation; they take food from farms and steal electric current. The wisest among them, like Nicodemus, are dissatisfied with this state of affairs, realising that they don't really have a proper society of their own at all; they're just really living off human resources, and that if anything were to happen to the humans, the rats would go under too:
We're like fleas living on a dog's back. If the dog drowns, the fleas drown, too.
Therefore the rats have decided to remove to a remote rural area and try to plant their own seeds and grow their own crops. This is the Plan - 'to live without stealing,' as Justin remarks to Mrs Frisby. It is vitally important to the rats as it represents their attempt to create an enduring and worthy society of their own.
The rats' vision, as embodied in the Plan, is one that is often shared by humans as well: the idea of retreating from modern society and technology, living in harmony with nature and being truly self-sufficient.