"Welcome to Tiankawi - shining pearl of human civilization and a safe haven for those fleeing civil unrest. Or at least, that's how it first appears. But in the semi-flooded city, humans are, quite literally, on top: peering down from shining towers and aerial walkways on the fathomfolk - sirens, seawitches, kelpies and kappas - who live in the polluted waters below. For half-siren Mira, promotion to captain of the border guard means an opportunity...