This was a surprisingly effective entry, in spite of there being not much of a story here. It mostly concerns the guys trying to avoid and/or thwart the attempts of a superstrong robut to destroy them. Why does it want to do this? What happened to it up in space? Did some sort of hostile intelligence reprogram it? These questions are never answered, though there's some speculation. The familiar attitudes are trotted out here -- we are taught once again to beware overenthusiastic creator scientists, and to value caution and suspicion of that which is not human; and I have to wonder if this didn't seem a little hackneyed even when this originally aired. But still the tension and danger is ratcheted up quite nicely, and there's even a larger than usual body count in this episode. I'm always impressed how such a limited environment--a submarine--can be used to create suspense so effectively. It's interesting to contrast this with the previous episode, about an intelligent and cunning "alien" individual out to harm the Seaview and by implication the human race: here the danger is much the same, though the menace is mechanical and silent and single-minded. But it's just as effective and I enjoyed it almost as much.