The Enterprise crew finds a devastated planet with only two survivors. The crew's attempts to convince the two to leave their doomed planet are consistently interrupted by the mysterious appearance of an unknown enemy ship. Meanwhile, Troi is bed-ridden by a psychic audio hallucination of music, music that matches a music box found at the survivors' house. Eventually one of the survivors, Kevin, admits that he is an all-powerful alien who has been causing the enemy ship to appear and the music to play in Troi's head, all to keep from being bothered and/or found out for what he really is. He tells the story of the planet's destruction, and how in a fit of revenge he killed off the entire species that murdered his lover along with the planet. This he regrets, and he had thought to live out his days alone with her specter, created by himself. Picard decides to leave Kevin to his self-exile.
There's something about that kind of power that makes the moral question presented intriguing. As a species, we're often struggling to keep our moral compass up-to-date with our newest powers, usually technology. We, too, have weapons that we're reluctant to ever use. Nuclear Holocaust, any one? But this goes beyond that because WWIII would probably harm the very people who unleash such weapons. Kevin's powers are more precise, so he experiences no physical repercussions, even when its a whole species that he wipes out. What would keep us from blowing our enemies to smithereens if we could be sure that we'd suffer no physical harm in return? Whenever we're powerful enough to annihilate someone else, there's usually something like resource dependence, or even powerful third parties, to keep us at bay. What if you could kill your enemy with a thought?
Well, I suppose first you'd enter a truce with the rest of your all-powerful species not to use such powers on each other. And then, if you have a lot of you running around, it could get quite messy if every one is always acting on a whim. You'd think Q would be more careful, eh? But Kevin is different than Q in that he seems to have a conscience quite similar to ours, or what we hope ours to be. He made a mistake and he regrets it. He doesn't like to play god. That makes him a likeable character, and therefore makes this a likeable episode.