Somewhere on the crumbling road between the Palisades Enclave in California and the remains of the Chicago Spaceport, a brightly-painted wagon rumbles along on the last rubber tires in the world. Mordecai Lehrer is moving east. His wagon is a traveling medicine show, a peddler's pack, home from part-time magicians—and a courier service for Earth's last enclaves of scientific knowledge. Ninety per cent of Earth's population perished in the Death, and the remaining ten blame the evils of godless science for that terrible plague. They would destroy the science-enclaves if they could—and they grow bolder day by day.
But the enclaves know what the people cannot: scientists in the Mars colony have discovered that the plague will inevitably be followed by a mutant form virulent enough to wipe out all of the survivors of the first siege. Mars has also found a vaccine, and even now a suicidal mission of mercy is racing toward the ruins of the Chicago Spaceport.
Meanwhile Mordecai Lehrer bumps across the plains of the west, carrying precious instructions from the California enclave on how to grow and use the vaccine. He travels in secret, and in fear, for all around him are the people whose lives will be saved if he succeeds—and who would kill him gladly to ensure his failure.