Stardate 3211.7: The Enterprise arrives at Gamma II to check out an automated facility and Kirk, Uhura, and Chekov prepare to beam down. However, before Scotty ca beam down, they instantly dematerialize from the transporter and find themselves on an alien planet with three suns. They're unable to establish communications with the ship, and several alien warriors move out to attack them. They try to use their phasers but they malfunction and they're forced into hand-to-hand. They're quickly overwhelmed and Kirk is downed by a female warrior, Shahna.
Galt, the master Thrall of the planet Triskelion, arrives to welcome them and they are all given collars to wear. Galt explains that they are now Thralls kept by the Providers, who run the planet. The Enterprise crew try to make a break for it but Galt's eyes glow and the collars choke them.
The Enterprise searches for their missing comrades without success, but Spock believes that something has happened to them since there is no trace even of their dispersal in the transporter.
The Thralls bring food for the Enterprise part and the male, Lars, announces that he has been selected for Uhura. She fights him off while Kirk's drill instructor, Shahna, arrives and informs him it is time for the nourishment interval.
Ensign Harris detects an ionization trail leading out of the system and seeing no other alternative, Spock orders them to follow the trail.
While Chekov's instructor Tamoon awkwardly makes romantic advances to him, Kirk explains to Shahna what "beautiful" means. She reveals that she was born on Triskelion and her parents were gladiators. They're interrupted when the exercise interval begins. A thrall is brought out for punishment and everyone is ordered to use him as a practice target. First Uhura and then the others refuse and Galt orders Uhura to take the thrall's punishment. Kirk assumes responsibility for Uhura's actions and Galt orders him to be practice target. The savage thrall, Kloog, attacks Kirk with a whip and chases him around the arena. With some advice from Shahna, Kirk manages to knock him down and free his hands, then choke him unconscious. The Providers finally speak, bidding on the newcomers until Provider #1 purchases them. Their collars are marked with the Provider's color and Galt wants that they will now be punished with death.
While the Enterprise continues along the trail, Shahna takes Kirk on an exercise run past some ruins. Kirk wonders if the ruins belonged to the Providers, then tries to sway Shahna to his side by talking about love. She backs away in fear so he tries to ask her about the Providers and she explains they aren't humans. However, her collar activates, choking her, and Kirk takes responsibility. Provider #1 finally ends the punishment but warns that Kirk must learn obedience. She's impressed that he would risk punishment on her behalf and they kiss.
The concerned McCoy and Scotty question Spock's conclusion and his pursuit of the ionization trail, but he convinces them to let him pursue the search and if they find nothing, to go back to Gamma II.
Back in his cell, Kirk knocks out Shahna. Lars is gone to report Uhura and Chekov has tied up Tamoon. They try to find Galt but he intercepts them and uses the collars to subdue them. Meanwhile, the Enterprise arrives in orbit and detects one concentration of life forms. Spock and McCoy prepare to go down alone but the three Providers take control of their ship and put them in contact with the landing party. Kirk explains the situation hen taunts the Providers into teleporting him to their underground base. The Providers are simply brains, and claim they are mentally superior. They inform him the Enterprise will be destroyed and its fate remains a mystery, while the crew is brought down as Thralls. Kirk challenges them with a wager: his people will defeat an equal number and if they win, the Providers will free the Enterprise and raise the Thralls to be self-sufficient. If Kirk loses, his people will willingly comply. The Providers agree but force Kirk to offer three-to-one odds, with him representing the crew.
Kirk has no choice but to agree and is teleported into the ring where he fights Lars, Kloog, and an Andorian. Kirk spears Kloog, then baits the Andorian into throwing his weapon and killing Lars. Kirk takes out the last Andorian but without a kill, Shahna is sent in to take his place. Furious, she prepares to kill him but hesitates and Kirk brings her down and forces her to yield. The Providers honor their wager and Shahna is eager to go with them, but Kirk says she has to learn from the Providers before she can travel to the stars.