'That Which Survives' is a good, low-key, atmospheric third-season episode that has a melancholy air to it, especially in retrospect after you learn what happened to Losira and her people. I would definitely rank it as one of the better shows of the last season, though admittedly the bar is pretty low. If there's anything really odd about 'That Which Survives,' it's the dialogue. All of the characters are really snappish with each other. Sulu will say something and Kirk will almost bite his head off. Same thing goes on back at the Enterprise; Spock is more of a smartass than we've ever seen him before. Almost anything anyone says to him is met with a response dripping in sarcasm. It's quite a different approach and frankly it becomes annoying after a while, though some of his lines are funny. I don't recall any other episode of ST with dialogue quite like this. And it's all the more peculiar as the writer, John Meredyth Lucas, was a Trek veteran who wrote four shows including the fine episode 'The Changeling.' None of his other scripts display this odd approach to dialogue. Lee Meriwether is a good choice for the mysterious, lonely Losira and the scenes on the planet are the most interesting.