Executive producer: Gene Roddenberry
Associate producers: Edward K. Milkis and Gregg Peters
Producer Fred Freiberger
Co-producer: Robert H. Justman
Story consultant: Arthur H. Singer

Story by: David Gerrold and Oliver Crawford
Writer: Margaret Armen
Dir.: Jud Taylor

Captain Kirk: William Shatner
Science Officer, First Officer, Cmdr. Spock: Leonard Nimoy
Dr. Leonard "Bones" Mc Coy: DeForest Kelly
Lt. Sulu: George Takei
Lieutenant Commander Montgomery Scott: James Doohan
Lieutenant Uhura: Nichelle Nichols
Plasus: Jeff Corey
Droxine: Diana Ewing
Vanna: Charlene Polite
Anka: Fred Williamson

Stardate 5818.4

The Enterprise is on her way to the planet Ardena, which is known for it's rich amounts of the rare element Zienite, to receive a shipment of the material, which would make an end to an epidemic disease on another planet. When Kirk and Spock beam down they are involved in a social struggle between the Troglytes, who are working in the mines, and the Stratos dwellers, the townspeople who rule the planet. Instead of getting the shipment they are attacked by the Troglytes. They are led by Vanna, who has served in the cloud city and obtained knowledge about the difference of living conditions. 

The Troglytes flee when 4 city dwellers are beamed down. They kill one of the Troglytes. The city–dwellers complain about the savage existence of the Troglytes and the High Adviser of the city, Plasus, points out to them that the Troglytes are inferior by nature and cannot be educated. Kirk and Spock are guests in the city until the Zienite is found. Here Plasus shows them pieces of art and introduces his daughter, Droxine, as his masterpiece. She looks at Spock and graciously says "It's this first time that I meet a Vulcan," "And for me, - Spock answers, - it is the first time I meet a masterpiece." Plasus offers his guests a room to rest.

While Spock gets acquainted with Droxine, the captain is attacked by Vanna, who tries to take him hostage. Kirk fights her and calls Spock. He arrives with Droxine, who has her arrested and explains to both that the Troglytes are not used to art, sun or warmth or anything in the city. Now Spock tries to convince her that all humanoids can learn and have part in the achievements they are working for. Droxine denies. When Spock and Kirk hear a cry they rush out and witness how Vanna is tortured with rays to reveal the names of the other Troglytes who are part of the mutiny – just after Droxine told Kirk and Spock that they in the city have no part in violence, as the Troglytes have. Trying to make Plasus aware that he uses torture, Spock and Kirk get into a discussion which makes obvious that the city dwellers cannot understand their approach to see the Troglytes as equal human beings who are fit to learn. 

Plasus orders them back to the ship. By now the time to avert a disaster on the epidemic stricken planet comes closer- they have only got 12 hours to avoid the dying of the people. On board McCoy has examined the Zienite and found out that it sets free a gas, which makes people act strangely. The gas makes people violent, too. Spock suggests that the brain seems to act normal again, after it has not exposed to the gas for a while, like Vanna's who had served on Stratos for a longer period. Gas masks could solve the problem, but Plasus does not believe them. In spite of Plasus' order to stay away from the planet, Kirk beams down to convince Vanna that the gas causes the discrepancy of development. 

They need the Zienite and he frees Vanna who pretends to believe him. When they are in the cave, Vanna has him taken prisoner and work as they do. Soon Kirk experiences the effect of the gas. He overwhelms Vanna, takes the communicator and orders Plasus to be beamed down into the cave, the exit of which he has blocked with the help of the phaser. As the three are exposed to the gas all feel the it after some time. As Kirk and Plasus fight Vanna takes the communicator and has them all beamed on board. She is convinced now that the masks will help. The Zienite provided and in hope to change the mental condition for her people she looks forward to a better future. Plasus is not happy to think of changes and more chances given to the Troglytes, but agrees in certain terms. Droxine, too shares with Spock her intention to visit the caves and the Troglytes not to close her eyes any more from things happening on the planet.