instructor said.

"Where's Billy Kasker?" Susan Sidwell suddenly asked.

The group halted. Billy Kasker was no longer following them. A little stir of consternation ran through them as they realized the class president was missing.

"Billy! Billy!" the instructor called.

There was no answer.

"I just don't understand this. He knows he should remain with us."

"Maybe some of these horrible natives grabbed him!" Susan Sidwell said. The group was startled—and suddenly afraid.

The instructor took a deep breath. "I have a Thor gun. I'll go find him. Joe, you are in charge of the group until I return. All of you remain in the middle of the street and don't move."

The instructor went back along the street. He was exasperated and a little alarmed. If anything happened to Billy, how could he explain the matter to the gate captain or to Billy's parents?

"Billy!" he called again and again.

Suddenly he had an answer from an alley.

"Here, sir—here I am. Are you looking for me? I'm sorry, sir." Billy himself appeared in the alley.

Reassured at the sight of the youth, but angry, the instructor moved into the alley. "What is the meaning of this? You have alarmed all of us."

"I'm awfully sorry, sir. But I saw something back here that interested me, and I stopped to take a look. I hope you will forgive me." His manner was so contrite and his chagrin so complete that the instructor had no choice but to forgive him.

"Of course, Billy. But you mustn't do anything like this again. It might be dangerous."

"I won't, sir. I promise. But I wonder, since you are here, if you would be good enough to explain to me the thing I saw back here. It will only take a minute."

"What is it?"

"It's something in one of the houses. I came back looking at something else, then caught a glimpse of this. If you will come into the back yard you can see it. I would really like to have you explain it to me, sir. You are always so clear in your explanations." Billy Kasker's manner was very winning.

"Well, if it will only take a minute—" The instructor followed Billy into the back yard. At the rear was a shed with an open window. A plot of grass separated the shed from the house. On the second floor of the house, a window had been shattered.

"There's something up there in that broken window. If you will come here, sir, you can see it better."

"Um. Ah! Oh, yes." The instructor's back was to the open window of the shed. He stared upward at the house.

Two brown-coated arms came out of the window of the shed and clamped a fierce grip around his throat, jerking him backward against the wall. He grabbed frantically for the Thor gun.

The face of the brown native appeared in the window of the shed. "Get that gun, Billy!"

Billy Kasker was already in action. He snatched the gun from the instructor's flailing hands.

The brown native leaned from the window. Muscles bulging in his powerful arms, he lifted the instructor upward and through the window. A thump came from inside the shed. Billy Kasker, Thor gun ready for use, went through the door.

The instructor was writhing on the floor. The native had a knee on his chest, a knife in his hand.

"This is for the race you think you've conquered!" the native said. He plunged the knife into the instructor's throat. Green liquid spurted from the wound.

"Green blood!" the native said. "One of the chemical differences." He came to his feet. The dying instructor was forgotten. The native's hand went out. "Billy, am I glad to see you. I was afraid you wouldn't recognize me in spite of the tune I was whistling as I walked past you on the street."

"I wouldn't forget," Billy Kasker said.

"But, Billy, it's been twelve years since I traded you, as a kid of five, for one of their brats—changing the bracelet as I changed you. Many times since then I've thought you had forgotten, or that I wouldn't live to see the