Tom Swift Jr. realizes one of his greatest ambitions when the U. S. space agency commissions him to do a comet probe. With his latest invention, the telesampler, Tom hopes to glean unknown scientific facts from the next comet to pass near earth -- and even, perhaps, make a manned landing. But his erstwhile enemies the Brungarians are a threat to the daring research project and U. S. scientific prestige.

Meanwhile, weird unidentified flying objects blazing across earth's skies are causing the U. S. Department of Defense great concern. When Professor Feng, of the University of Heildelberg, gives Tom a strange clue to the UFO's -- a clue that dates back to the Middle Ages -- the young space scientist is soon caught up in a web of danger and intrigue.

How Tom succeeds in solving the complex riddle of the UFO's -- just in time for a desperate race with a deadly comet as earth's fate hangs in the balance -- makes one of the most tension-packed, thrilling stories in the Tom Swift series.