Red and Blue are players on opposite sides of a war that spans all of time and multiple interwoven universe-threads. They travel up and down stream, stopping wars or starting them, saving the lives of those who will later invent vaccines or new mathematics, or preventing people who would be influential from ever being born. Neither Red or Blue are human, though they frequently take on human form during their missions. Red works for the Agency, a technology future, whose agents skip and jump lightly across spacetime in hundreds of small interventions, causing miracles and murders. Blue works for the Garden, a future based in natural biology, which favors a slow long-game approach to change, often embedding agents for the full course of a human life in key historical moments. Equal and opposite, Red and Blue begin a correspondence, initially threatening and taunting, but which soon turns philosophical and poetic. The discovery of this relationship by either the Agency or the Garden would ensure the writer's death, but they keep writing, though each letter is equally a trap and a lifeline. Is this love, or sabotage? Can it be both at the same time? I listened to this four hour audiobook all in one day and though the story has some weaknesses I enjoyed it immensely.