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 to help her downtrodden people. But if earning the trust and respect of her human colleagues wasn't hard enough, everything Mira has worked towards is put in jeopardy when Nami, a know-it-all water dragon - fathomfolk royalty - is exiled to the city. When extremists sabotage the annual boat race, violence erupts, as does the clampdown on fathomfolk rights.
Both Nami and Mira must decide if the cost of change is worth it, or, if Tiankawi should be left to drown.
