The Cloud Minders

The planet Merek II is gripped by a botanical plague. The only cure is the mineral zeenite, and the only practical source of that is Ardana. But Ardana has a stratified city: the troglytes who labor live in harsh conditions, while philosophers and artists enjoy the beauty of its floating city, Stratos. One among the troglytes, Vanna, means to change this, and plans to exploit the crisis - and Federation representatives Kirk and Spock - to accomplish that.