Having survived a firefight with Cthonians below, Rex and Aral flee back up to Ætheria. As soon as they arrive, the two are separated and interrogated. Facing his government's refusal to accept the truth about Cthonia, Rex embarks on a quest to find Aral and rescue her from torture and imprisonment by the ever-more-paranoid Head Ductor, the ruler of Ætheria. Fortunately for them, a member of the militaristic Ætherian Cover Force decides to turn his back on the Head Ductor. "Hypoxia" is book #4 from "Vertical World", an EPIC Press series, a division of ABDO.