"So, describe the symptoms of this plague that ravaged your planet in the past."
The Enterprise must find out the truth about a plague when McCoy is accused of starting it, which gets complicated when they are infected.
This episode provides examples of:
- Clear Their Name: McCoy is accused of being responsible for a plague that wiped out much of a planet. McCoy angsts about the possibility that it is unintentionally his fault. It's the rest of the Enterprise crew who are adamant that he is completely innocent and spend most of the episode searching for proof.
- Determined Doctor: McCoy immediately tells Spock to take him back to the Enterprise to find a cure for the plague despite the high odds against him (only he and Spock are still fully capable) and Spock's reminder about the reality that he'll contract the plague and die as well if he fails.
- Fainting: Kirk does something akin to the exhaustion- or anemia-based variety when The Plague kicks in. Luckily, Spock is there to catch him.
- Find the Cure!: McCoy and Spock have to discover a cure for a disease which is killing the Enterprise crew.
- The Immune: Spock cannot catch the disease.
- Pre-emptive Declaration: When a native from the planet that put McCoy on trial follows them.Kirk: Besides, he'll be sure to take advantage when he sees that we've carelessly left our shuttle bay door open.
Uhura: But we didn't, sir.
Kirk: Oh yes. See to that little oversight, will you, Mr. Sulu? - Red Herring: The skin color changes apparently associated with the plague are actually caused by aurora radiation. The correct diagnosis and solution are found by ignoring it and analyzing the other symptoms.
- Self-Destruct Mechanism: As per General Order Six, the Enterprise will self-destruct in twenty-four hours if the entire crew dies, presumably to prevent the disease from spreading.
- Would Rather Suffer: After another fight with Spock, McCoy tells Kirk that if he lands in jail again, Kirk should leave him there rather than sending Spock to effect another jailbreak.
