The Federation starship Enterprise inspects an uncrewed station at Gamma II. Captain Kirk, Communications Officer Lieutenant Uhura, and navigator Ensign Chekov attempt to transport but vanish before the system activates. Commander Spock orders a search, discovering an ion trail.

Kirk, Uhura, and Chekov find themselves in a gladiator arena on an unfamiliar planet, where they are attacked by four humanoids and subdued. Galt, Master Thrall of Triskelion, informs them they will be trained to entertain the Providers and fitted with "collars of obedience." Uhura, Chekov, and Kirk are assigned individual "drill thralls": Lars, Tamoon, and Shahna, respectively. The Providers bid for the new thralls using "quatloos."

During a run outside the arena, Kirk tries to learn about the Providers from Shahna, but her collar activates when she talks too freely. Kirk argues that he should be punished, and when they return to their cells, Shahna expresses gratitude. As she embraces him, Kirk knocks her out and frees himself, Uhura, and Chekov with her key, but Galt stops them.

The Enterprise follows the ion trail to a planet with a humanoid settlement. As Spock and Dr. McCoy prepare to beam down, Provider One warns them not to transport. Kirk's voice updates them, and he challenges the Providers to reveal themselves, discovering they are three disembodied brains. Kirk proposes a wager: if he and his officers win a fight against an equal number of thralls, the Providers will teach the thralls self-governance. If they lose, the Enterprise crew will become thralls. The Providers agree, but Kirk must battle three thralls alone.

The Enterprise crew watches from the ship as Kirk defeats two thralls, injures a third, and subdues Shahna. The Providers declare Kirk, the winner and unlock the thralls' collars. Shahna wants to join Kirk in space, but he says she and the other thralls must first learn to live in their newfound freedom.