On stardate 8264, Kagg's Kluggoth was in port at Hephaestus for shore leave and contact with the native Hephaestan government. Kagg encountered a fellow visitor to the world, Rear Admiral James T. Kirk, and informed Kirk of his intention to win a treaty with the Hephaestans for the purpose of gaining mining rights on the world. Unbeknownst to Kirk and his USS Enterprise crew, Kagg had learned of a Hephaestan secret technology artificially augmenting their civilization's mental capabilities. To the end of discovering how to utilize the secret for his own personal gain, or perhaps the gain of the Klingon Empire, Kagg had decided to extract the secret from the Hephaestans. Initially, Kagg had his crew attack Hephaestan individuals, and eventually the Klingons begin murdering them for the secret of a symbiont chip implanted in their nervous systems. When the microchips were revealed by an autopsy performed by Leonard McCoy and Christine Chapel on a murder victim, Kagg panicked and set off a tense standoff situation, eventually holding Joanna McCoy hostage in an underground cavern. Kagg died when a large explosion wiped out the area after the Enterprise crew and Joanna beamed away. Joanna's fiance Suvak was also killed in the blast, having helped to thwart Kagg's plans.
In a captain's log entry, Admiral Kirk theorized that Kagg must have been insane, after Kagg's crew fled shortly after their commander's death and the Klingon authorities claimed no knowledge of his unlikely plan, and agreed to pay reparations to the Hephaestans for the deaths incurred. (TOS comic: "All the Infinite Ways")