Enterprise is en route to the New Paris colony to deliver much needed medical supplies, needed to counter a plague. It passes Murasaki 312, a quasar-like system. Since there is time, Kirk's standing orders require him to dispatch a scientific team to study the phenomenon. The expedition immediately runs into trouble, pulled of course and forced to an emergency landing on Taurus II, then beset by the hulking, homicidal natives of that planet. Can Spock's purely logical approach to leadership save the expedition and reunite its members with Enterprise?

Enterprise stops at Cestus III, there to visit Commodore Travers, who enthusiastically invites Kirk to bring his key people to the surface. Beaming down, Kirk discovers the outpost has been destroyed! The landing party comes under attack, but Kirk and Spock recover a powerful ballistic weapon and strike back. The aliens flee, with Enterprise in hot pursuit, into an unknown region of space. There, both ships are paralyzed by an "unidentifiable power" from the distant star system of the Metrons, and both captains are transported to a world where they may engage in hand to hand combat to determine the fate of their respective ships. But is that the whole story?