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Thann and Lal were two Vians who travelled to the doomed Minara system, whose sun was about to go nova. There, they abducted the two members of a Federation observation post, Linke and Ozaba, and tortured them until they died, although Thann insisted they were not killed by the Vians but by their own fears. When Captain Kirk, Commander Spock and Doctor McCoy came to investigate, Thann and Lal abducted them as well and imprisoned them with Gem, a mute woman with the ability to heal wounds at the cost of feeling the pain of them herself. Observing their interaction, Thann noted the three newcomers had a love of life, which was the prime ingredient of their test.
Telling Kirk that they only needed one subject, which he was willing to agree to if they let his friends go, Thann and Lal transported him away and tortured him, while keeping the others prisoner. Returning him to his friends, they announced they would need either Spock or McCoy for further experimentation. With Kirk having passed out from his injuries, McCoy rendered Spock unconscious and volunteered to go with them.
By the time Kirk, Spock and Gem tracked McCoy down, Thann and Lal had tortured McCoy to near death. They imprisoned Kirk and Spock in a forcefield to stop them interfering and revealed the reason for their actions: They had the power to save one of the system's populations from death and were testing Gem to see if her people deserved to survive. They had arranged the whole thing to see if she would be willing to sacrifice herself to heal McCoy. Gem was willing but McCoy refused to let her die for him. Spock managed to pass through the Vians' forcefield through emotional control, disarming Thann and freeing Kirk. Kirk urged the two Vians to show the compassion they claimed to prize, returning their devices to them. Lal healed McCoy, after which he and Thann departed with Gem.