Time travel may still be impossible, but the Chronoscope, sensing the nearly imponderable tracks of ancient photons, allows a glimpse into any event in Earth's distant past. Historian Arnold Potterley is rebuffed when he petitions for use of the Chronoscope to study ancient Carthage. So he commissions the building of a private time-viewing machine, with unexpected results.

In the far future, the material needs of humankind are tended by the omnipresent Machine, which makes their environment, in vast tunnels beneath the earth, comfortable and safe. But safe from what? And what if the Machine Stops? A young boy, Kuno, finds his questions can't be answered until he has lifted the veil of security and trespassed upon the surface.