 In order to save an inhabited planet from becoming inviable by a botanical plague eliminating all oxygen production, the Enterprise goes pick up zenite, the mineral cure, on the only known planet where it is mined. When Kirk and Spock beam down, they are attacked by  troglites  (troglodyte miners) but not captured, high council adviser Plasus intervenes and brings them to Stratos, the luxurious metropolis, which literally floats in the sky. However it is under attack from  disruptors , troglite revolutionaries who want equal life conditions for their peers, who do all the hard work, mainly toiling miserably in mines and dwell in caves on the barren surface below. Dr. McCoy finds out the alleged intellectual inferiority of the troglites is not genetic but the reversible effect of exposure to the invisible gas emitted by the zenite...