So Doctor Nordenfeld had been exposed to any contagion Kathy could spread. He hadn't been protected at all. Yet the potted plants on Kathy's side of the barrier were marked with great white splotches which grew almost as one looked, while the botanical specimens in the doctor's part of the hospital--as much infected as Kathy's could have been, by failure of the ship's mechanic to build the sterilizer to work two ways: the stacked plants, the alien plants, the strange plants from seventy light-years beyond Regulus--they were vividly green. There was no trace of chlorophage on them. Yet they had been as thoroughly exposed as Doctor Nordenfeld himself! The doctor's hands shook. His eyes burned. He took out a surgeon's scalpel and ripped the plastic partition from floor to ceiling. Kathy watched interestedly. "Why did you do that, Doctor Nordenfeld?" she asked. He said in an emotionless, unnatural voice, "I'm going to do something that it was very stupid of me not to do before. It should have been done when you were six years old, Kathy. It should have been done on Kamerun, and after that on Altaira. Now we're going to do it here. You can help me." * * * * * The _Star Queen_ had floated out of overdrive long enough to throw all distance computations off. But she swung about, and swam back, and presently she was not too far from the world where she was now many days overdue. Lift-ships started up from the planet's surface. But the _Star Queen_ ordered them back. "Get your spaceport health officer on the vision-phone," ordered the _Star Queen's_ skipper. "We've had chlorophage on board." There was panic. Even at a distance of a hundred thousand miles, chlorophage could strike stark terror into anybody. But presently the image of the spaceport health officer appeared on the _Star Queen's_ screen. "We're not landing," said Doctor Nordenfeld. "There's almost certainly an outbreak of chlorophage on Altaira, and we're going back to do something about it. It got on our ship with passengers from there. We've whipped it, but we may need some help." The image of the health officer aground was a mask of horror for seconds after Nordenfeld's last statement. Then his expression became incredulous, though still horrified. "We came on to here," said Doctor Nordenfeld, "to get you to send word by the first other ship to the Patrol that a quarantine has to be set up on Altaira, and we need to be inspected for recovery from chlorophage infection. And we need to pass on, officially, the discovery that whipped the contagion on this ship. We were carrying botanical specimens to Cassim and we discovered that they were immune to chlorophage. That's absurd, of course. Their green coloring is the same substance as in plants under Sol-type suns anywhere. They couldn't be immune to chlorophage. So there had to be something else." "Was--was there?" asked the health officer.