The prologue states that an extraterrestrial armada (later known as 'Grakks') having destroyed sixty percent of the human population, a society of godlike 'Benefactors' rebuild Earthly civilization, in exchange for military help against the attackers. To replace the missing sixty percent, the Benefactors bestow humanlike intelligence and habits upon Earth's gorillas, and thus produce a combined civilization on Earth.
The prologue describes an extraterrestrial invasion (later called 'Grakks') that decimated 60% of humanity, leading to a society of godlike beings known as 'Benefactors' rebuilding civilization in exchange for military assistance against the invaders. To compensate for the lost population, the Benefactors bestow human-like intelligence and behaviors upon Earth's gorillas, resulting in a combined civilization on Earth.