From the introduction: Humanity is constantly evolving, but it faces two possible futures. In one, humanity grows in spirit and ascends to become the fae, creatures of light, happiness, and creation. In the fae future, good has prevailed: society has matured; war is unheard of; and so on. In the other future, humanity grows in hatred and descends to become the undead, creatures of dark, fear, and destruction. In the undead future, evil has prevailed; society is selfish; peace is unheard of; and so on. Thanks to the quirks of quantum reality, both timelines have already happened--yet that is not guaranteed. Timelines are fluid. Fae and undead existence could simply cease to have happened at any time. The only way to guarantee that a future is safe is to change the past--to go back in time to before humanity evolved and push history towards that future. That’s why both fae and undead agents travel back to the same past. It can go either way, and thanks to the agents’ meddling, it will. You are not just fighting for good or evil. You are fighting for your entire existence.