During an exploration of a distant planet, three members of the Federation starship, USS Enterprise, were assaulted, with their red blood cells wholly drained of hemoglobin. Captain Kirk, recalling a frightening entity he had met eleven years back while on the USS Farragut, decides to skip his meeting with the USS Yorktown to go after the killer. Despite the attempts of the Enterprise crew to detect the creature using their onboard sensors, it attacked another landing party, taking the hemoglobin out of two more members. When Kirk discovers that security officer Garrovick has not used his phaser on the creature, he removes the ensign from his post.

The Chief Medical Officer, Dr. McCoy, upon examining the reports of Kirk's experience, challenges the captain's fixation with the alien form. As a junior officer, Kirk had hung back when shooting phasers at the being, causing the demise of half the Farragut company. Although his courage was rewarded with an accolade, the captain kept blaming himself for the fatalities. Kirk insists that the entity is an imminent menace. Ensign Chekov interrupts to inform that the entity from the cloud is withdrawing from the planet.

The Enterprise tracked the entity until it reversed its course and headed towards the vessel. Kirk commanded his crew to launch their weapons, but they had no effect. The being then went through the shields and into the ventilation system. Spock pointed out to his captain that he should not feel remorse for his previous encounter with the creature since their conventional arms could not harm it. To counter this irrational guilt response from Kirk, Spock was trying to convince Garrovick that there was nothing wrong with his actions when the creature emerged from a vent in Garrovick's chamber.

Having made Garrovick leave, Spock attempts to block the entity from entering, but his green blood, composed of copper, repels it. Knowing that neither he nor Garrovick could have hurt the being, Kirk orders the ensign to return to work.

Eventually, the creature departs the vessel. Thinking it is making its way to the Tycho system to reproduce, Kirk and Garrovick teleport down with an antimatter bomb. With the entity about to embrace them, Kirk and Garrovick teleport away, and the explosive detonates, obliterating the creature.