Her crew still recovering from a recent shipwide outbreak of choriocytosis, the Enterprise is en route to the dedication ceremonies for Deneb V's new Academy of Sciences. On the way, Commander Spock succumbs to the very disease from which the rest of the crew has already recovered. Because Vulcans have copper-based blood, choriocytosis — which afflicts humans no more severely than does pneumonia — will prove fatal to Spock, unless a supply of a counteragent known as strobolin can be found within three days' time.
The closest source of the drug is the planet Beta Canopis, which is four days away at maximum warp. However, Kirk arranges to have the U.S.S. Potemkin pick up some strobolin and hand it off to the freighter S.S. Huron, which is headed in the Enterprise's direction. Unfortunately, an unknown vessel attacks the Huron before the rendezvous can take place, and relieves the freighter of her cargo of dilithium and strobolin. The Huron's Captain O'Shea doesn't know the identity of the attackers, but Kirk immediately sets out to discover it, while Spock grows ever weaker from the effects of his advancing choriocytosis.
Following the marauding ship's unique radiation trail, the Enterprise soon catches up with the culprit: an Orion vessel. After some bargaining with the Orions, Kirk and the Orion captain agree to meet on a nearby asteroid, where the pirate leader has consented to hand over the strobolin. Though Kirk is aware that he's walking into a trap, Spock's deteriorating condition leaves him little choice. The Orions secretly plan to double-cross Kirk by detonating a dilithium bomb on the geologically unstable asteroid, but Chief Engineer Scott beams the dilithium out of the device in the nick of time, and beams Kirk and the Orions aboard the Enterprise as well. Captured and prevented from taking his suicide pill — Orion operatives are supposed to kill themselves rather than be captured — the Orion captain and his crew now must face trial for the crime of piracy. Spock recovers from choriocytosis, thanks to the recovered strobolin, although he must endure Dr. McCoy's good-natured (and very relieved) jibes about his green Vulcan blood turning out to be a liability for once.
This work contains examples of:
- Alien Blood: As McCoy says, for once Spock's Vulcan heritage makes him more vulnerable to a threat than his human crewmates. While iron based hemoglobin can fight off the blood-based disease spreading through the ship comparatively easily, green Vulcan blood is way more susceptible.McCoy: It's a nuisance to humans. To Vulcans it's fatal. Ninety-three percent probability, as Spock would say.
- Aw, Look! They Really Do Love Each Other: McCoy is genuinely devastated when it seems he won't be able to save Spock. Of course, once they get the cure and Spock begins to recover, the pair go back to snarking at each other again.
- Badass in Distress: Spock, whose illness kicks off the plot. While most of the action is going on, he's lying comatose in the sickbay.
- "Could Have Avoided This!" Plot: From the viewpoint of the Orions. If they had opened communications with the Huron crew and been told that the latter was about to rendezvous with a heavy cruiser, they'd probably have cleared the area in seconds. Orions Know When to Fold 'Em, thank you very much. Even after that, if they had just taken the dilithium (which was what they were after anyway) and left the strobolin (useless to them), Kirk wouldn't have been so desperate to capture them and would likely have ceased pursuit when they entered the Asteroid Thicket.
- Cyanide Pill: Once he is captured and beamed aboard the Enterprise, the Orion captain tries to take one, only to be prevented by two random Redshirts grabbing hold of him in time. Evidently, they have learned from their previous encounter with Orions.
- Fainting: The first indication that something is seriously wrong with Spock is when he collapses on the Bridge. Of course, being Spock, he wouldn't have shown any sign of weakness till it became that serious.
- Find the Cure!: Sort of. At first it is comparatively straightforward, just a question of arranging a rendezvous with another ship carrying the cure, but when Orion pirates attack the latter, things get a whole lot more complicated.
- Insulted Awake: An increasingly desperate McCoy attempts this with a comatose Spock, "praying for a comforting insult". Of course, it doesn't work, nor did the doctor really expect it to.McCoy: Blasted Vulcan. Why couldn't you have red blood like any normal human?
- O.O.C. Is Serious Business: For once, Scotty doesn't protest when Kirk asks him to increase the speed.Scotty: Aye sir, I'll see what I can do.
