Once free of the cages, SK540 had no difficulty in leading his troop out of the building. The door of the laboratory was locked, but a window was slightly open from the top. Rats can climb up or down. Like a silver ribbon they flowed along the dark street, SK540, looking exactly like all the rest, at their head. Only one person in the deserted streets seems to have noticed them, and he did not understand the nature of the phenomenon. * * * * * Young Mr. and Mrs. Philip Vinson started housekeeping in what had once been a mansion. It was now a rundown eyesore. It had belonged to Norah Vinson's great-aunt Martha, who had left it to her in her will. The estate was in litigation, but the executor had permitted the Vinsons to settle down in the house, though they weren't allowed yet to sell it. It had no modern conveniences, and was full of rooms they couldn't use and heavy old-fashioned furniture; but it was solidly built and near the laboratory where he worked as a technician, and they could live rent-free until they could sell the house and use the money to buy a real home. "Something funny happened in the lab last night," Philip reported, watching Norah struggle with dinner on the massive coal-stove. "Somebody broke in and stole about half our experimental animals. And they got our pride and joy." "The famous SK540?" Norah asked. "The same. Actually, it wasn't a break-in. It must have been an inside job. The cages were open but there were no signs of breaking and entering. We're all under suspicion till they find out who-dunit." Norah looked alarmed. "You too? What on earth would anybody want with a lot of laboratory rats? They aren't worth anything, are they--financially, I mean?" "Not a cent. That's why I'm sure one of the clean-up kids must have done it. Probably wanted them for pets. They're all tame, of course, not like wild rats--though they can bite like wild rats if they want to. Some of the ones missing are treated, and some are controls. It would just be a nuisance if they hadn't taken SK540. Now they've got to find him, or do about five years' work over again, without any assurance of as great a success. To say nothing of letting our super-rat loose on the world." "What on earth could even a super-rat do that would matter--to human beings, I mean?" "Nobody knows. Maybe that's what we're going to find out." * * * * * That night Norah woke suddenly with a loud scream. Philip got the gas lighted--there was no electricity in the old house--and held her shaking body in his arms. She found her breath at last long enough to sob: "It was a rat! A rat ran right over my face!" "You're dreaming, darling. It's because I told you about the theft at the lab. There couldn't be rats in this place. It's too solidly built, from the basement up." He finally got her to sleep again, but he lay awake for a long time, listening. Nothing happened.