delivers medical supplies to the planet Dramia. Kirk, Spock, and McCoy meet with the Supreme Prefect and his council and have a final meeting once the supplies are delivered. However, the Supreme Prefect asks for a moment of their time and has Commander Demos present a warrant for the arrest of Dr. McCoy. He stands accused of spreading a plague that killed hundreds of colonists on Dramia II 19 years ago. The warrant is approved by the Federation and they have no choice but to turn McCoy over. Kirk and Spock go to the Hall of Justice where Demos explains that McCoy headed an inoculation program and after he left, the plague struck. Kirk is leery of Dramian justice, believing the trial will be a kangaroo court. They meet with McCoy, who admits he isn't sure. Kirk is confident of his innocence and prepares to leave for Dramia II to insure. Once they're gone, the security guard assures McCoy that Dramian justice is swift, and notifies his superiors. Demos prepares to depart in a scout ship.
As the Enterprise
proceeds to Dramia II under Kirk's personal authority, Spock reviews the historical record and notices the plague cause pigmentation changes in the victims, and Vulcans have a natural immunity. They pick up Demos on the sensors and Kirk leaves the hangar doors open. Demos takes the bait and lands, Kirk and Spock immediately take him into custody. Demos demands they notify the Federation and Kirk says he will, eventually, and impounds Demos meanwhile. Demos realizes he has been tricked but there's nothing he can do about it.
arrives at Dramia II, passing through a radioactive but non-lethal aurora. Kirk, Spock, and Demos beam down to the planet. They spot one survivor and they chase after the Dramian. They follow him into a crevasse and Kirk opts to go in. The survivor attacks Kirk out of fear, although Demos claims it's because Kirk is an Earthling the same as McCoy. Demos explains that a few that were away at the time returned after the plague, and chose to live there in desolation. Another Dramian arrives, Kol-Tai, and says he survived. He describes the progression of the plague, as victims turned first blue, then green, and then finally red in the final stages. Kol-Tai remembers McCoy, noting he treated him for Saurian Virus and insists that McCoy wouldn't kill.
Kirk, Spock, Demos, and Kol-Tai arrive and they proceed back to Dramia through the aurora to present Kol-Tai as a witness on McCoy's behalf. Kol-Tai is taken to Sickbay but as they approach Dramia, Spock reports that Kol-Tai is turning blue from the plague. They order a quarantine but it's too late: Kirk has turned blue. The plague spreads throughout the ship, infecting everyone except Spock. They go into orbit above Dramia but everyone is in a weakened state. Spock orders General Order Six: the ship will self-destruct 24 hours after everyone on board is dead. He contacts the Supreme Prefect and informs him of the situation. He asks for McCoy to be released to board the ship and treat the illness. The Supreme Prefect refuses to hear him out since Demos cannot testify. As Kirk turns green, Spock informs him he'll beam down on his own to rescue McCoy. Spock beams down into the prison facility and knocks out the security guard. McCoy initially refuses to leave, insisting he have a trial to determine his innocence. Spock informs him of the plague and McCoy agrees to go with him despite the fact he'll die if he doesn't find an antidote.
Kirk beams them aboard before collapsing. Spock and McCoy go over the records and can find no records of a similar plague. When the aurora interferes with McCoy's readings, Kirk manages to gasp out that the aurora is responsible. Spock analyzes the disease without the color change symptoms caused by the unique aurora and comes up with a cure. McCoy remembers there was a similar aurora 19 years ago, which means he's innocence. The problem is that there's no known antidote. However, Spock points out that McCoy treated Kol-Tai for Saurian Virus, and he survived the first onslaught of the plague. McCoy uses the Saurian Virus antibodies in his laboratory to make an antidote and the crew recovers. Demos and Kol-Tai express their thanks and the Dramians award McCoy a medal for his medical research. The Supreme Prefect agrees to ignore McCoy's unorthodox release and they agree to ignore Demos stowing away. Back aboard the ship, Spock notes that McCoy has fallen behind on the crew's vitamin treatments and things go back to normal.