“Oh. You’ve been out of touch. Well, you haven’t missed any news from Callisto, because we’ve had an efficient news blanket in operation for almost a year and a half. And for a while it was a voluntary one—just about two years ago, when the air started going bad. We didn’t want outsiders to know.”

I blinked. “The air?” In a dome-city like this, the air supply was, of course, wholly artificial, and its proper maintenance was of vital importance to the entire community. “What happened to the air?” I asked.

“I’m not sure,” she said. “None of us are. Suddenly it became impure. People began sickening by the hundreds; some died, and almost everyone else was ill in one way or another. A tremendous investigation was held by the people who were our government then—Cleve Coldridge was our mayor, a fine man—and nothing could be determined about the source of the impurities. And then my father—he’s dead now—invented this.” She tapped the metal collar she wore around her throat.

“And what, may I ask, is that collar?”

“It’s a filter,” she said. “When the collar is worn, it counteracts the impurities in the air, through some process I don’t understand. My father died shortly after he developed it, and so he didn’t get a chance to offer it to the public. He willed the design and the process to three—friends—of his.” Her mouth clamped together bitterly, and I saw her struggling to fight back tears. Almost automatically, I put my arm around her.

“I’ll be all right,” she said. “Every time I think of those three, and what they’ve done to Dad’s invention—”

“Tell me about it later, if you want.”

“No. You might as well know the whole story. The three of them—Martin Hawkins, an Earthman, Ku Sui, a Martian, and Kolgar Novin, a Venusian—announced my father’s device to the public as if they had discovered it themselves. It was the solution to our air-impurity problem. They started turning out the collars in mass production, and within a month everyone in Callisto City was wearing one.”