Fifteen years after warring artificial intelligences nearly destroyed Earth, Abby, the daughter of Killday hero Leah Gibson, finds an artifact from that struggle, upsetting a delicate balance of power and dragging her into the middle of a new fight for humanity's survival.

One AI faction is working with humanity to repair a biosphere teetering on the edge of collapse, while another faction cares only about elevating itself to a higher plane of intelligence and will destroy anyone who gets in the way. As the only humans still not controlled by AIs race to build huge orbital habitats in space, a more secretive organization grows in the shadows and idolizes the man who triggered the nano-replicator attacks that nearly destroyed the world. They, too, believe the only way to stop the AIs is to annihilate everything.

In Nebula Award winner William Ledbetter's Level Six, one woman has the power to save humanity - if she can survive long enough to use it.