An android and exorcist, Takiya Naru, plunges alone into the horrific mysteries of the Halcyon Abyss. The Abyss is a space station entirely filled with water, originally constructed as a marine habitat imitating the abyssal deeps of alien oceans, a place to preserve potentially useful aquatic monsters brought from war-torn and dying extrasolar worlds. But in the four hundred years since its abandonment, much has changed beneath the unquiet surface of that black sea, and Takiya's lonely dive will reveal what horrors have been left to fester and multiply in the depths where no light falls, horrors far stranger and more sophisticated than mere beasts.