An ordeal staged by two Vians beneath the surface of Minara II, who tortured Kirk and McCoy to learn whether the mute empath Gem's people deserved rescue from their dying sun.
In the Original Series episode The Empath, the Enterprise arrives to evacuate a research station before the Minaran star goes nova. Kirk, Spock and McCoy find the station empty and are pulled into an underground laboratory, where they meet a mute woman McCoy names Gem and two Vians, Lal and Thann.
The station's two scientists, Linke and Ozaba, have already died in the Vians' experiments. Gem can take others' injuries into her own body, and the Vians want to see whether she will risk her life to do it. They torture Kirk, then allow one of the others to be chosen; McCoy sedates his friends and takes the place himself.
The Vians can save the people of only one planet in the system. Gem's willingness to heal the dying McCoy, and Kirk's rebuke that the Vians themselves have forgotten compassion, settle the matter in her world's favour.