Star Trek - The Original Series, Vol. 23, Episodes 45 & 46: A Private Little War/ The Gamesters of Triskelion
"A Private Little War," Ep. 45 - Kirk and company beam down to a primitive planet visited by Kirk 13 years ago. But some of the once-peaceful natives now have sophisticated weapons, courtesy of the Klingons. "The Gamesters of Triskelion," Ep. 46 - Kirk, Uhura and Chekov are enslaved on the planet Triskelion. There they are trained in the art of combat so that their unseen masters, the "Providers," can make wagers on the outcome.