Kirk, Chekov and Uhura step onto the transporter pad in order to beam down to an unmanned astrogation station on Gamma II for a routine inspection but vanish before the transporter can be activated. They are instantly transported to a fenced-in arena and are surrounded by four humanoids of differing origins. Finding their phasers to be useless they fight hand to hand but are finally overcome by the humanoids.

A being called Galt appears, dressed in black, and congratulates the Enterprise team on their performance and spirit. He explains that he works for the 'Providers' as Master Thrall of Triskelion and that Kirk, Uhura and Chekov will spend the rest of their lives as thralls. They are fitted with obedience collars and led towards some cells. On board the ship Scotty can find no trace of a transporter malfunction and Spock is unable to determine where the landing party may have been sent. Sensor scans confirm that they are not in the same solar system as the Enterprise.

On Triskelion, Kirk and the others try to make a run for it but Galt's strange eyes light up, activating their collars and bringing them to their knees in pain. In their separate cells, the Enterprise crewmembers are introduced to their drill thralls. Chekov's is Tamoon, a kind of cat woman who obviously takes a fancy to him. Uhura gets Lars, a handsome bronzed male and Kirk's drill thrall is Shahna, a gorgeous humanoid female with a huge upswept hairstyle and an exotic, tight-fitting silver costume. She explains to him that it is their job to train them and, once completed, they will be sold off to the highest bidder among the Providers. Kirk tells her she is beautiful but she does not understand what he means. When the 'auction' is over Galt tells them that they now belong to Provider One and that any disobedience will be punishable by death.

Shahna takes Kirk on a training run. They stop for a rest and Kirk asks her about the Providers but she is unable, or unwilling, to answer. He tells her about other planets, civilisations and freedom but when she softens and does start to talk about the Providers her collar activates, making her collapse with the pain. Kirk shouts into the air that he is responsible and her pain suddenly stops. Amazed at Kirk's efforts to help her she responds warmly when he kisses her but Galt appears from nowhere and orders them back to the cells.

On the Enterprise Spock has located a faint ionisation trail and decides to follow it, despite Scotty and McCoy's protests about leaving the system, as no other logical alternative presents itself. Back in the cells Kirk takes advantage of Shahna's confusion over her feelings for him and knocks her out. He frees the other two and they manage to make their way into the arena before Galt appears and activates their collars. The Enterprise has followed the ionisation trail all the way to Triskelion and Spock prepares to beam down with McCoy. Before they can proceed further they are stopped by the Providers who take control of the ship's functions. Kirk somehow hears what is going on and, hoping he can be heard, explains the situation to Spock.

The Providers transport Kirk to their secret chamber where he discovers that they are all disembodied brains. They explain to him that they lead lives of pure intellect and that gambling on the thrall contests, using inferior beings, is their only means of entertainment. They plan to kill Kirk and the others, who are proving to be too troublesome, but they are persuaded by Kirk to accept a wager that he and the others can beat an equal number of thralls. If they win they go free and the Providers will free the remaining thralls and train them in matters of civilisation. If they lose, Kirk pledges that the crew of the Enterprise will stay and compete in further contests for their entertainment.

The Providers accept, but on their own terms - only Kirk will fight - alone against three thralls. He has no choice but to accept their terms and is transported back to the arena. The fight begins and Kirk eventually manages to kill two of the thralls and injure the third. The Providers halt the contest and replace the injured thrall with Shahna. She nearly manages to beat a tired Kirk but he eventually overcomes her and she surrenders. The Providers keep their word and free the remaining thralls. Shahna and Kirk share a lingering kiss before he returns to 'the lights in the sky'.