Put simply, the Plan of the Rats of NIMH is "to live without stealing." It was originated by Nicodemus who had grown discontented with the idea that rats always lived by stealing from others, especially humans. He wanted to see if the Rats could become self-sufficient. The Plan (it is known simply as the Plan among the Rats) is what prompted the Rats' move from their home under the Fitzgibbon's rosebush, where they stole not only food, but running water and electricity, to Thorn Valley, where they would live by subsistence farming
The Plan's main detractor was Jenner who cynically believed that, if the Rats are smart enough to do so, why shouldn't they steal from humans?