"Calling space ship RQ257!" he grated into the transmitter. "Interplanetary Patrol just behind you. Pilot, please identify yourself! Do you know who was aboard the experimental rocket X911, that was just destroyed?" A few seconds later he heard a dazed, grief-anguished voice speaking in response: "Yes ... I ought to know. I came out to watch our test of the Energy Barrage Penetrator, which we thought would be successful. I am Paul Arnold. The man who was just killed was John Arnold, my father." John Arnold! Yes, Harwich had often seen photographs of this daring, hawk-faced old student of the Forbidden Moon in the scientific journals. He had been the greatest of them all! But there wasn't much to do for him now but shrug ironically, and report the nature of his death by radio to the Interplanetary Patrol Base on Ganymede, largest of Jupiter's satellites. "I'm sorry, Paul Arnold," the patrol man told his informant in sincere sympathy. "Thank you," the quavering voice of Paul Arnold returned. "And now, if you don't mind, I've got to get back to Ganymede City. Dad's gone, but I've got to carry on his work." * * * * * Harwich didn't meet Paul Arnold, the son of the dead scientist, face to face for more than a month, Earthtime. But on patrol duty out there in the lonely reaches of the void, with the stars and the roar of his rocket motors for company, he saw a good deal of the leering, greyish sphere of Io. It seemed to taunt him with its masked secrets, hanging so near to the tremendously greater bulk of Jupiter. But the Forbidden Moon told him nothing new at all. Through his binoculars he saw the deserts and hills and those supposed ruins. Near the equator was something that looked like a vast, pointed tower. But Harwich had seen this before, often. Something moved near the tower now and then, as on other occasions. But maybe this distant movement was only the shifting of clouds of dust, blown by a thin, frigid wind, in a tenuous atmosphere. Then, back in Ganymede City, came that meeting with Paul Arnold. It happened at the Spacemen's Haven. Evan Harwich, on furlough now, was sipping Martian _kasarki_ at the bar. Presently a hand was laid on his arm. He turned to face a slight-built youngster, who could not have been more than eightteen. But his peculiar gold-flecked eyes were as distant and scared and bright as if they had seen Hell itself. "You're Harwich," said the boy. "I'm Arnold. They pointed you out to me as the patrol pilot who reported my father's death. I wanted to talk to you. I don't know just why, except that you were there too, when Dad was killed. You saw what happened. And people have told me that you were a square shooter, Harwich."