I heard low whispering coming from them as they stowed their spacesuits in dull-green lockers ranged along the side of the airlock, and headed back toward their trucks. They were all looking at me, and obviously they were commenting on the fact that I didn’t have any collar. They seemed shocked at that, and very worried.

“What’s this collar business?” I asked the driver of my truck, as we moved through the inner lock and into the city proper.

“You’ll find out, chum. Just make sure you can run fast when they spot you, though.”

“When who spots me?”

“The guards, dope. The Tax Agents. You don’t think you can breathe for free on Callisto, do you?”

“You mean they tax your breathing?” I asked, incredulously, and before I could get an answer I saw a cordon of guards forming around our truck.

There were half a dozen of them, burly men in blue uniforms, all of them wearing the ubiquitous metal collar. They had halted our truck, which had been last in the procession. I saw the other trucks in the convoy rolling on toward their destination somewhere in the city.

“Don’t make trouble for me,” my driver said pleadingly. “I’ll be docked if I don’t get my cargo back on time.”

One of the men in uniform reached up and opened the cab of the truck. “Come on out of there, you.”

“Who, me?” I asked innocently. “What for?”

“Don’t play games,” he snapped. “Get out of that truck.” He waved a lethal-looking blaster at me, and I decided not to argue with it. I leaped lightly to the ground, and as I did so the uniformed man signalled to my driver that he could go ahead.

The six men ringed threateningly around me. “Who are you?” the leader demanded. “Where’d you come from?”

“That doesn’t matter,” I said belligerently. He put his hand on my arm, and I jerked away. “I’m a tourist. Want to see my landing permit?”