On Saybrook's planet, all life is one unified organism, like the cells of a human body. Plants produce fruit to be eaten by the--exclusively vegetarian--animals; rats and insects keep their population at a sustainable level; even bacteria do not multiply so much as to cause disease. And every living thing from the plants and animals down to the tiniest microbe has not eyes but two little green patches of fur, sense organs that bind them into the unified life form.
When Saybrook's ship discovered the planet, it seemed a paradise--lush and filled with life, and no violent competition for food or territory from the animals. But then the truth was discovered: the white lab rats they had brought along were all pregnant. And when they gave birth, the baby rats had little patches of green fur in place of their eyes. The other animals aboard became pregnant as well--and the human women. Rather than contaminate Earth with this viral unity, Saybrook transmitted a report and vaporized his own ship.
Now another ship has come, carefully prepared to protect against the life on Saybrook's planet, and carrying many female animals to detect any breach in that protection. With their investigation completed, they begin to return home, since all seems well, yet Dr. Weiss is suspicious... but how can he destroy the ship and all its crew on mere suspicion? But the reader knows that his suspicion is correct--they have a small stowaway, a missionary for the unified life on Saybrook's planet, just waiting to welcome a new world.