Kirk faces a decision about arming a primitive people when the Klingons already have.
Kirk returns to a planet he surveyed years earlier as a young officer and finds that the Klingons have secretly supplied flintlock rifles to the villagers, upsetting a peaceful balance with the hill people led by his friend Tyree. After Kirk is bitten by a mugato, a venomous ape-like beast, Tyree's wife Nona, a healer, saves him with a local root and ritual.
Kirk decides to give Tyree's people matching weapons to restore the balance, a choice McCoy strongly opposes. Nona, who wants power, betrays the group and is killed. The episode is widely read as a commentary on the Vietnam War.