The ruins were getting younger.They were thought to be the ruins of an invadingforce of space monsters that men had defeatedduring the Dark Ages centuries before. Butthe ruins were visibly getting newer -rebuilding themselves.

The militarists who had reconstructed societyafter the supposed invasion were getting panicky.Until they found a complete invader vehicle -and learned it travelled through time.But what was Time? What was Now?Could there be more than one Time Front - onegoing forward, one in reverse? And what wouldhappen when two such fronts met in theinevitable COLLISION COURSE?