The book is set in the not-so-distant future, where humankind has all but succeeded in controlling all life on the planet and almost completely wiping out all insect life. The earth is divided into a "Green Zone" which humans totally dominate (or so they believe) and a diminishing "Red Zone" that is not yet conquered.

The "Green Brain" of the title is an intelligent organism that embodies and arises from nature's resistance to human domination.
It is able to command social insects to form humanoid-shaped collective organisms which it uses to infiltrate the "Green Zone".

The book is about a small team sent into the jungles of Brazil to investigate the problem, who find out that some of their assumptions were wrong.


In an overpopulated world seeking living room in the jungles, the International Ecological Organization was systematically exterminating the voracious insects which made these areas uninhabitable. 
Using deadly foamal bombs and newly developed vibration weapons, men like Joao Martinho and his co-workers fought to clear the green hell of the Mato Grosso.

But somehow those areas which had been completely cleared were becoming reinfested, despite the impenetrable vibration barriers. 
And tales came out of the jungles . . . of insects mutated to incredible sizes . . . of creatures who seemed to be men, but whose eyes gleamed with the chitinous sheen of insects. . . .

A fascinating examination of the fragile balance between consciousness, man and insect from one of the best-loved science fiction creators of all time