In one room Joe finds thousands of small black cubes. When he dozes off next to one he has a lucid dream of taking part in a weapons drill. He understands that the dream that has obsessed him since childhood must have come from a fragment of a similar cube that had been found in a Cro-Magnon cave with artifacts dated to 20,000 B.C. Sandy suggests looking for cube readers, reasoning that learning from a cube by sleeping near it is inefficient. They find a pair of reading helmets and quickly begin learning all about the fortress, its weapons, and an implacable Enemy with which the garrisonâs civilization may have been at war for 100,000 years or more. They also learn that the Enemy has sent a new attack, one that will rip apart the solar system in a matter of days.