Divided We Stand

While the Enterprise examines Friendship III, an Earth probe launched some 170 years earlier, an alien nanite infestation seizes the ship's computer. As Spock and Scotty fight to purge the systems, an explosion leaves Kirk and McCoy unconscious and infected. In a shared delusion the two friends wake in 1862 America, Kirk in Union blue and McCoy in Confederate grey, on opposite sides of the Battle of Antietam. McCoy is forced into battlefield surgery with little more than whiskey and a piece of wood for his patients to bite on, while Kirk refuses to fire on the enemy and grows close to Billy, a homesick young soldier, and the veteran Pappy. Aboard ship, Spock and Scotty clear the computer, but Dr M'Benga is alarmed to find the captain's body reacting to the dream. When a grenade wrecks Kirk's leg on the battlefield, McCoy has no choice but to amputate, and in sickbay the real limb begins to die. Spock lures the nanites out with a decoy, Chief Drake's prosthetic arm, and with M'Benga applies the same approach to the two patients. The cure works, but not before Kirk learns that young Billy has been killed, and both men return carrying the weight of a war they never truly fought.