During a mission, the Enterprise encountered an immense, highly sophisticated space vessel, the Fesarius, and its commander, Balok. Balok's appearance eventually yielded to Spock's determined efforts: a bluish, cat-eyed alien in an eerily-lit control center that wavered and rippled.
Balok condemned the crew as warlike because they had been forced to destroy a marker buoy. In fact, he had forced their hand, goading and threatening them with the device. Then, he advised the crew that they and their vessel, "obviously the product of a primitive and savage civilization," were to be destroyed.
A tense standoff followed, during which Captain Kirk sought vainly for a solution. Eventually, he bluffed Balok with corbomite – a ploy that apparently worked, for Balok decided not to immediately destroy the Enterprise. Instead, it would be towed to a planet of the First Federation, where the crew would be imprisoned and the Enterprise destroyed.
Kirk gambled again, as his ship was being dragged to its fate; he attempted to shear away from Balok's pilot vessel (a much smaller craft, launched from the Fesarius) and succeeded, apparently damaging Balok's ship in the process. At this point, Kirk could have attacked – certainly, he'd been provoked – or he could have fled. He chose instead to board the small vessel and offer aid. There, he learned the truth; "Balok," or at least the Balok he had seen, was merely a puppet.
Moving past this display, Kirk and his landing party discovered the real Balok, a diminutive alien. This Balok revealed that the entire encounter had been an elaborate test of character and ethics. The purpose of the puppet, "Mr. Hyde to my Jekyll", was to frighten and intimidate. Balok shared a drink of tranya with the landing party, and gave them a tour of his ship.
The Fesarius had a crew of only one, so Balok was the only representative of the First Federation seen, so far. Kirk left Crewman Dave Bailey with Balok, for an exchange of cultures and ideas. (TOS: "The Corbomite Maneuver")
Balok was portrayed by Clint Howard. Despite having a childlike appearance, Balok was meant to be an adult alien. For this reason, Howard's vocal track was deleted in post-production and the character was instead voiced by Walker Edmiston. (Star Trek: The Original Series 365, p. 34)
The Balok puppet was created by noted sculptor Wah Chang. (Star Trek: The Original Series 365, p. 35) John Fifer functioned as Balok's puppeteer. wbm The voice of the puppet Balok was supplied by Ted Cassidy. He was selected for the role because his deep vocalizations were deemed appropriate for the part. After the producers asked him to record the puppet's lines, Cassidy did so during post-production. (Star Trek: The Original Series 365, p. 34)
Clint Howard later reprised his role as a grown Balok, part of Comedy Central's 2006 roast of William Shatner. In it, Howard portrayed Balok as being an alcoholic, addicted to tranya. As with the episode, Balok's voice was again dubbed in by another actor.
The Balok puppet appears in the end credits of several other TOS episodes.