The Enterprise becomes involved in an important mining planet's class struggle in this installment of the enduringly popular science fiction series. Captain Kirk and Mr. Spock visit the planet Ardana on a routine mission to collect a shipment of an element known as zenite, when they are unexpectedly ambushed by several of the planet's miners, known as Troglytes. This attack awakens the Enterprise crew to the divided nature of Ardana's society, where the Troglytes toil and suffer in the zenite mines while the wealthy literally live in the clouds, making their home in a lavish floating city. When he discovers that the exploitation of the Troglytes is threatening their health and lives, an angered Kirk chooses to disrupt the planet's social order -- but will his interference create a more equal society or anarchy? ~ Judd Blaise, All Movie Guide