An old lighthouse, providing a beacon to ships for some six generations, has been declared a national landmark. However, it is being shut down because it is now a navigational hazard to airplanes. Unbeknownst to the Terraneans, an undersea race had been indirectly using the light to power their underwater city, and without it, they will die.

Interesting way of getting power: harness the kinetic energy of sea anemones opening and closing.

Also interesting that Frank, the lighthouse keeper, never noticed the cuts in the floor for the undersea elevator shaft.

These underwater races have a great penchant for jumping to conclusions, don't they? They've been parked under the lighthouse for who knows how many years, never coming up to pass the time of day with Frank, never letting ANYONE know of their presence, and yet, when the light is shut down, they instantly assume that the Terraneans are knowingly committing genocide.

Lucky that Frank thought to carry writing materials with him out to Stingray, or that fool Marina would never have let him in.

All this time on Stingray, and Marina's never learned how to turn on the lights? Too busy reading her magazines?

Speaking of the lights, from the angle at which they were set on Stingray, I don't see how they could have flashed enough light to effect the anemones. Actually, I thought that Frank's plan was to get on the radio, explain matters to Commander Shore, and ask him to temporarily return power to the lighthouse until they could make other arrangements.

Why didn't one of the guys from down below come up with Frank? That's another bad habit of these underwater races--setting unreasonable time limits.

Having jumped to the conclusion (there they go again!) that the Terraneans had tricked them and that their city was doomed...um, why didn't they just evacuate?

BTW, the lighthouse would have provided light in a complete radius. That underwater light would only have covered a segment of the area. Stingray should have brought at least one more beacon, possibly two or three. Also, Troy was petty enough to get back at his captors for the way they treated him. He could have fetched that beacon for them right away, but there was a party coming up, so it could wait....