Welcome to the Well World: a construct of an ancient defunct race known as the Markovians. The Well World acts both as the controller of and the gateway to 1560 worlds created by the Markovians at the end of their time.
Humanity is reeling when the Dreel (a viral, hive-minded species) attack the Milky Way and seem ready to take over the galaxy. In desperation they go back to the ancient sciences of the defunct race of Markovians which no one really understands.
But using science concepts as alien and as powerful as those of the Markovians can have dire consequences, as the humans discover when they inadvertently create a situation which can lead to the destruction of the universe itself.
The Well World can help prevent total destruction, but the planet- sized computer has itself been damaged and must be repaired before it can be used to save the universe. Nathan Brazil can fix the damage, but where is Nathan Brazil? It is up to Mavra Chang and Obie the sentient robot to find Brazil before the entire universe is destroyed.