Lt. Dave Bailey
Anthony Hall

Balok
Clint Howard

Voice of Balok
Vic Perrin

Voice of Balok puppet
Ted Cassidy

During a star mapping mission the Enterprise encounters a strange spinning cube which Captain Kirk is forced to destroy when its approach endangers the safety of the ship. Spock's speculation that it may have been a warning beacon of some kind is proved to be correct when a gigantic ship of unknown type appears. It identifies itself as being the Fesarius, a ship of the First Federation, commanded by captain Balok.

He accuses the crew of the Enterprise of being a savage species and announces his intention to destroy the starship in ten minutes, despite Kirk's offer to withdraw from the area. Spock describes the situation as being a 'checkmate' but Captain Kirk turns to another game, poker, to find a way of saving his ship and crew. He instructs Balok that the Enterprise has been equipped with a defensive weapon that uses a substance called 'Corbomite' that has the ability to reflect any weapon power directly back on the aggressor.

The ten-minute deadline passes and the bluff seems to have worked. However, instead of destroying the Enterprise, the Fesarius sends out a smaller craft to capture her in a tractor beam with the intention of towing Kirk and his crew to a prison planet. Using the full power of her warp engines, the Enterprise is able to break away from the smaller ship, causing severe damage to it in the process. Balok sends out a distress signal, so Kirk, McCoy and ship's navigator Bailey beam aboard the alien vessel to render assistance.

They discover that the alien image they have been communicating with on their view screen is just a dummy and that the real Balok is small and child-like in appearance. Balok discloses that the distress call was just a ruse to see what Kirk and his crew would do and, in view of their attempt to help, would allow them to survive for further examination. After negotiations, Bailey agrees to stay with the Fesarius to engage in a cultural exchange exercise.