Three disembodied beings called the Providers kidnapped Kirk, Uhura and Chekov to Triskelion to fight as gladiators until Kirk won a wager for the thralls' freedom.
In 2268 Kirk, Uhura and Chekov were transported from the Enterprise to the planet Triskelion. They were fitted with collars and trained as fighting thralls under the drill thrall Galt, while three brains known as the Providers placed bets on their combat. Kirk began winning over his trainer, Shahna.
The Enterprise tracked them across the light-years. Kirk then proposed a wager to the Providers: if he defeated three thralls in combat, the thralls would be freed and taught to govern themselves. Kirk won, and the Providers kept their word.