Note: Originally published as "Journey to Misenum" in "Startling Stories", August 1953. Expanded for book publication in an Ace Double in 1955.

Synopsis: From the first page of the Ace Double: "Invaders on time's hidden battlefield - On the universal time-track, this alternate Earth was two thousand years behind. While in our world and its parallel twins science was surging ahead, there it was still Ancient Rome. Yet though chariots rolled the streets, behind a secret wall crouched an ultra-modern army of flying tanks, atomic artillery, and tough American G.I.'s! For in this backward Earth there was a menace that had to be met and blocked... a stalking foe from a devastated world of the far future, a hungry, vicious, depraved planet seeking new lands to replace its own radioactive ruins! And the key to this fantastic battle between the past, the present, and the future was a lone American girl, Elspeth Marriner, the Time Watcher."