“Day of the Dove”
“Day of the Dove” The Enterprise picks up the survivors of a destroyed Klingon ship. Soon the situation runs out of control when part of the Enterprise crew and the Klingons are trapped in a part of the ship. They engage in endless skirmishes with ancient weapons which are absolutely pointless because all wounds heal immediately so as to maintain the balance of power. It turns out that an entity consuming hatred is catalyzing the fighting, and an agreement between Kirk and the Klingon captain Kang ends it. summary by Ex-Astris-Scientia
That was one hell of an episode in my humble opinion. First of all, helping those ugly Klingons is something I always like to see. The Federation is about help and compassion, not petty quarrels.
Secondly, reaching an agreement with Klingons, convincing them that violence… VIOLENCE! is wrong… heck! Good job, Captain Kirk.