As the episode starts, it appears that the Enterprise has successfully completed its mission to deliver medical supplies to the planet Dramia. However, after the leaders thank the Enterprise crew, they present Captain Kirk with an arrest warrant for Dr. McCoy for the crime of mass murder. Nineteen years earlier, McCoy had supervised a vaccination program on the planet Dramia II, but once he left the planet, a plague killed all inhabitants.
Since the warrant is legal, McCoy is taken into custody. With no other options, Kirk takes the Enterprise to Dramia II in an attempt to seek evidence regarding the source of the plague. They find a survivor and when they talk with him, they learn that he was off-planet when the plague started, and Dr. McCoy saved his life. The plague affects humans as well, although Spock is immune. Using broad interpretations of the law, Spock manages to free McCoy and he is able to find a cure. The episode ends with all charges dropped and peace is maintained between the Federation and Dramia.
The potential for introducing a plague when a new species visits a planet is a very real issue. Furthermore, that plague may not kill the natives, but be in the form of microbes that out-compete the native flora and fauna. The problem with this episode is that a plague that killed all the intelligent creatures on a planet would be big galactic news. It is impossible that such an event would be kept from the Federation for nineteen years and there would be a strong move to learn the cause. Finally, the cure was simple to find once the proper approach was taken. Way too easy, even for the miracle workers of the Enterprise.
