 Kirk, McCoy, Chekov and a security guard have beamed down to a planet where a 100 heads strong human colony reported an attack, but condition readings are normal, except no other people. Spock reports a Klingon ship approaches, after three years of truce with the Federation, but is suddenly disabled, drifting. The Klingon captain Kang, who also received a distress call, manages to beam down and capture Kirk s party, holding them responsible for his 400 dead crewmen, claiming the Enterprise instead, Kirk refuses; a shimmering, formless alien watches unnoticed when Chekov volunteers to be tortured first, claiming his brother -he has none- died in the colony. Kirk agrees to beam them aboard but uses an alert button, so Spock has the beaming doctored species-sensitive to delay and arrest the  guests ; the alien follows unseen, takes over the ship, gets it at warp nine and traps most crewmen. When Kirk s fist hits the Klingon captain, suddenly as by magic both sides hold swords appearing, which also fill the armory, of an unknown material, as what locks the crew in. Several men, even McCoy, get murderous delusions. Spock s sensor readings show both sides have 38 armed men, but there is one more life unit aboard- an alien that cannot even be pinned down, being energetic and intelligent, able to alter both matter and minds. Kang has taken over engineering and cuts life support for all human-held sections. The alien keeps causing surprises, without letting anyone die, but why? Kirk tries to convince the Klingons there s a worse enemy toying with both races...