Set out on another planet where a human spaceship lands for a research and they observer a huge mass moving below them. On closer look this mass appears to be humans, and the density of the population increases towards the sea. The reason for this is algae,the only nutrition found on the planet. On a beach they discover tall, huge humans (developed due to the natural selection), controlling the best access to the algae. Eventually the researchers die, because humans eat them.
A prototype faster-than-light spacecraft crewed by two men is sent to the Sirius system, known from robotic exploration to include an earthlike world. In orbit around the world, they notice that one of the continents has a thin, strange border all the way round its coastline, which radiates a low heat and appears black in visible light. After exploring the edges of the smaller continents, and discovering that the ocean hosts only one lifeform — a single species of algae that they think might have been genetically engineered — they decide to explore the large continent with the border.
When they discover just what the black border is — a seething mass of trapped humans with very dark skin, feeding off the algae and each other — the result is the death, by suicide, of one of the crew, and the self-destruction of the ship by the traumatized survivor upon his return to Earth — and a chilling reminder that there may be great danger waiting for further human explorers. The story ends on a hopeful note, with the project leader believing that Earth can help the humans at Sirius, and a fearful one, with the crewman then speculating that the humans were seeded by carnivorous aliens as food animals.