During one of its first trips, the Sea Venture takes on a parasite, an alien symbiont that can pass from one person to another, leaving them changed in subtle but consequential ways.

It was developed in part as an alternative to a more expensive orbital colony; one of the advocates of the space habitats, professor Paul Newland, is aboard to discover whether CV poses a threat to his dreams.

The sole doctor on board, Dr. Wallace McNulty, improvises an isolation ward, but cases keep occurring, several per day.

For instance, crew members decide that their jobs are pointless and abandon them; married people leave their spouses.

[1] The modifications wrought by the (apparently) moralistic symbionts echo the theme of Knight's earlier stories: "Rule Golden", "Natural State", and "The Dying Man", in which artificially increased empathy makes it impossible for people to be cruel without suffering the victim's pain.