As the Enterprise completes its mission to deliver medical supplies to Dramia, a surprising event occurs; the Dramians give Captain Kirk an arrest warrant for Dr. McCoy. According to them, 19 years prior, McCoy was responsible for a medical plague that killed much of the populace on Dramia II. At the Hall of Justice, Kirk and Spock learn that the plague followed a mass inoculation program which McCoy and his medical team had administered to cure the Saurian virus.

A few hours later, with McCoy being held in Dramian detention, Kirk takes the Enterprise to Dramia II to investigate the charges, and hopefully find evidence to exonerate his friend. Dramian justice can be swift and merciless, so Kirk feels he has little time to spare. Pursuing them, the hostile Dramian security chief Commander Demos sneaks aboard, but he's soon tricked by Kirk and Spock, and his one-man craft is impounded. Near Dramia II, the ship passes through a sparkling aurora, then Kirk, Spock, and Demos beam down to the planet's surface.

They chase — then are attacked by — a Dramian who lives among the city ruins, disconsolate in his grief. Subduing him, they soon meet Kol-Tai, the single survivor of the virus. He agrees to testify on behalf of McCoy, and they return to the ship. However, on the return trip to Dramia, not only does Kol-Tai turn blue, but so does Demos and every crew member except Spock. His comrades incapacitated, Spock is given command of the Enterprise and places it on general quarantine. He appeals to the Dramian Supreme Prefect to release McCoy to find an antidote, but the Prefect refuses.

Alone, with his shipmates beginning the second stages of the plague, Spock beams down to the planet and breaks McCoy out of the detention center. On Enterprise, they exhaust every lead, until chancing on the realization that the orbiting color-changing aurora was present before the precipitation of both plagues. Theorizing it is the aurora and not the plague that causes the color change, McCoy uses antibodies from the virus he cured 19 years ago to save the crew and the Dramians. Later, the Dramians drop all charges and give McCoy an award for his significant achievements in the field of interstellar medicine.