@mawr It is beautifully told and wonderful story... and I don't think I could do justice to it by summarizing.
The story is told from the perspective of letters exchanged by two agents of diametrically opposed futures fighting across thousands of time lines and course of all human history.
Over the course of their rivalry they slowly start to fall in love with each other despite never meeting, only walking through echos of course of events they've taken to thwart each other.