A city above the clouds has a class system that depends on a mineral gas that makes the underclass dull.
The Enterprise arrives on Ardana to obtain zenite, which is needed to stop a plague. The planet is divided between the leisured people who live in the sky city of Stratos and the labourers, the Troglytes, who work in the mines. Kirk and Spock discover that the working classes are being kept down by an unseen gas.
The story is a simple allegory of class inequality, with the beautiful city in the clouds contrasted with the dark mines. It features Kirk using a gas-filtering device to demonstrate the real cause of the Troglytes' condition. The social commentary is plain, and the episode is remembered for its visual contrast.