On its way to deliver medical supplies to plague-ridden Makus III, the U.S.S. Enterprise passes Murasaki 312. Since they are under Starfleet orders to inspect galactic phenomena such as this quasarlike star group, McCoy, Spock, Scott and four crewmen take a shuttlecraft, the Galileo, for a closer look.
Without warning, the shuttlecraft is pulled off course and out of sensor range of the U.S.S. Enterprise. Spock manages to crash land the shuttle on a foggy, rocky area of Taurus II which is inhabited by giant, hostile humanoids. Between attacks by the war-like residents and quarrels among themselves, the crew attempts to repair the shuttle and get off the planet.
Because of an ionization of its instruments, the U.S.S. Enterprise is unable to find the craft. On board, Commissioner Ferris demands that Kirk abandon the search and proceed to Makus III. Two of the crewmen from the Galileo are killed by the giant humanoids and Lt. Boma demands, irrationally, that Spock stop his repairs in order to bury the dead men. Spock's logic will not permit him to comply with the lieutenant's wishes in the face of their present situation.
As the U.S.S. Enterprise prepares to leave the area without finding the missing shuttle, the Galileo manages an unsteady orbit around the planet, but is unable to pull away from its gravitational pull. Knowing it will send them crashing to the planet's surface if his plan fails, Spock ignites the remaining fuel. The U.S.S. Enterprise sees the meteor-like flare of the burning fuel and beams the crew aboard as the Galileo disintegrates in the planet's atmosphere. Later, Spock stubbornly insists that his desperate act was not a human instinct to gamble, but a logical Vulcan approach to their problem.