The Pirates of Orion

Bound for Deneb V, the Enterprise faces a medical emergency when Spock falls ill with choriocytosis. The disease is normally treatable, but Spock's copper-based Vulcan blood makes it deadly for him, and McCoy can only slow its progress. A freighter, the Huron, is sent to meet the ship with the drug he needs, but Orion pirates attack it and steal its cargo, which includes valuable dilithium crystals. With Spock's time running out, Kirk finds the crippled Huron and then tracks down the raiders. The Orions, anxious to protect their planet's official neutrality, will not admit to the attack. Kirk agrees to meet their captain on an unstable asteroid, only to discover that the Orion is carrying explosives and means to destroy both ships rather than be exposed. Kirk's crew defuses the threat, the pirate captain surrenders to face trial instead of taking his own life, and the stolen drug is recovered in time to save Spock.