The big orange strain-bubble over the Di-tube generator exploded like an A-bomb. Concussion rocked the room, shattering windows and piling brasshats and technicians together in aimless heaps. Dora and I landed together in a corner, knocking over the little cabinet that housed our first model Di-tube generator. The big generator overloaded, pouring out black smoke. Relays slammed like triphammers, shutting off current. The Di-tube shaft in the floor blinked out, taking the Twisters with it. But another Dimension-tunnel speared downward through the roof, ending at the spot where the exploded strain-bubble had been. _And it wasn't ours._ It was packed solid with what looked like bedsprings, coiled and conical, eight feet long and blazing with a crackling bluish light that hurt the eyes. They slid down the beam and popped out into the lab with a swift precision that meant they knew exactly what they were doing, to cluster like sizzling blue glow-worms about the smoking generator. Half-dazed as I was, the truth hit me with a shock that curled my hair like an Angora rug. Those sputtering fire-worms were not ganging around the dead generator just to get warm. They were _studying_ it. I got up somehow and headed for the laboratory exit, carrying Dora's limp form in my arms. The military beat me to it. The generals and their staffs were already there, jamming the doorway like a herd of stampeded sheep. I was the last man out, leaving the fire-worms still circling about the smoking generator. Doc Maxey had stopped on the graveled gyro court halfway between the lab and his house on the hill above but he wasn't waiting for us. He was staring back toward the lab with his scrubby beard fluttering in the night wind and his eyes shining like wet marbles. I looked back to see smoke pouring out of the laboratory windows. Great sparks of light shot out through it but it wasn't until I understood Doc's yammering that I knew what they were. "They've left the rift!" he was yelling. "They're free--we've let those monsters loose on the world!" The fire-worms scattered systematically, taking off in every direction like snaky skyrockets. One minute the air was full of them--the next they were gone. "This new Di-tube came through when our strain-bubble burst," I said, looking up at the shaft of vacancy dwindling away into the sky. Dora came out of her faint then but I didn't put her down--I liked holding her too well. "But how, Doc, and from where?" "From superspace, the plane of existence immediately above ours," Doc said, wringing his hands. "The energy backlash from our generator must have weakened the spatial barrier between dimensions, enabling them to break through at the point of stress." Even I could understand that. Dora got it too and hid her face against my chest. "We've got to stop them somehow," she said. "Jerry don't just stand here--_do_ something!"