A mind-opening collection of short science fiction from one of the genre's most revered Grand Masters. Legendary author Robert A. Heinlein proclaimed, "To read science fiction is to read Simak. A reader who does not like Simak stories does not like science fiction at all." The remarkably talented Clifford D. Simak was able to ground his vast imagination in reality, and then introduce audiences to fantastical worlds and concepts they could instantly and completely dig into, comprehend, and enjoy. In the title story, a man's newfound ability to walk in the past allows him to dwell among dinosaurs, saber-toothed tigers … and something even more timeless. In "Construction Shack," the first manned expedition to Pluto reveals that no matter how advanced aliens may be, even they don't always get everything right. And in "Univac 2200," the thin line between humans creating technology and humans becoming technology is about to be crossed—and there may be no going back.