Karnas was one of the tribal chieftains on Mordan IV. He took the crew and passengers of a Federation ship hostage and demanded weapons for their release, intending to take revenge on the other tribes for the death of his father. Starfleet officer Mark Jameson was sent to negotiate and, without telling his superiors, secured the release of the hostages by giving Karnas the weapons he demanded. He then provided the same weapons to Karnas' rivals in order to honour the spirit of the Prime Directive. As a result, what Karnas had intended to be a swift victory turned into a civil war that lasted forty years.
Nevertheless, Karnas eventually gained victory over his enemies and became the planet's leader. However, he blamed Jameson, now a Starfleet admiral, for his suffering and sought revenge. He took the staff of the Federation embassy hostage and blamed it on dissidents, requesting Jameson's presence as a negotiator. Jameson arrived on board the Enterprise and contacted Karnas, quickly realising the truth of the situation.
After a failed attempt to rescue the hostages, Jameson beamed down with Picard and Crusher to meet with Karnas. Jameson had taken drugs that had restored his youth and Karnas initially refused to believe the young man was his old rival, but was convinced by Picard showing him the stages of Jameson's rejuvenation and Jameson revealing a scar Karnas gave him as a pact. Karnas' first instinct was to try and kill Jameson, but on learning he was already dying from the side effects of the drug, he simply stood and watched him die in pain. "Rest, Jameson. Your long night and mine are over,"he remarked, before ordering the hostages released.