During a routine transport to a planet, Captain Kirk, Lieutenant Uhura and Ensign Chekov are intercepted and abducted by a powerful and distant transporter beam. Arriving, literally, flat on their backs on a planet called Triskelion in the M-24 Alpha star system, the are immediately attacked by beings from several different species, each of them wearing a collar around their necks.
Kirk and his crew mates have been brought to this planet to fight as gladiators in combat games in order to entertain and provide exciting gambling for the disembodied entities called the "Providers." Each assigned a "drill thrall" of their own, Kirk and his companions are collared as well, which asphyxiate those who are disobedient to the Providers.
On board the U.S.S. Enterprise, Spock, aware that the crew members are missing, attempts to locate them, and eventually finds the origin of the beam. Upon its arrival, the crew of the starship is also taken captive by the Providers.
With the ship and his entire crew at stake, Kirk makes a final wager to the Providers which they cannot resist: himself against three drill thralls on the condition that if he wins, all the drill thralls go free and are taught how to live for themselves again. If Kirk fails, he promises himself and the entire crew as the most entertaining fighters they've ever seen.
Kirk beats the three drill thralls and wins the wager, and the Providers set all the thralls free and permit the U.S.S. Enterprise and her crew to leave also.