 The Enterpise finds the USS Exeter in a fix orbit, its over 400 crew members physically reduced to crystals. An officer left a recording, a warning to leave as quick as possible, but Kirk, Spock, McCoy and a security guard beam down to the planet, where they find a Turkic-looking Kuohm people about to execute by ax a prisoner of the so-called Yangs, which the Exeters captain Tracey declares impossible savages; he states to be his crew only survivor because of an unidentified natural immunizing factor on the planet against the fatal affection which dates from a biological war centuries ago, so they seem stranded there forever, but the immunization also allows the Kuohms to live many centuries. Then Yangs attack, without fear of death, numerous and almost unstoppable. When Spock points out to Kirk Tracey must be charged for violation of the primary directive by arming the Kuohms with phasers, he has the Kuohms imprisons them, because Kirk won t drop the charges in view of the Yang danger puts him in a cell with a couple, which attack him and escape. McCoy finds there is no serum, just natural selection by rare survivals and resistance by antibodies which simply take a while to enter the system, everyone can leave, yet Tracey refuses and tries to force Kirk to have phasers beamed down. After the escapee leads the Yang hordes to the Kuohm village, everyone is captured, but remarkable cultural parallels with earth open possibilities...