Ace raised her blaster. "You've already killed me once, girl," Kreer said. "Didn't you learn anything from that?"

When Bernice asks to see the dawn of the space age, the Doctor takes the TARDIS to the United States of America in 1957 — and into the midst of distrust and paranoia. The Cold War is raging, bringing the world to the brink of atomic destruction.

But the threat facing America is far more deadly than Communist Russia. The militaristic Tzun Confederacy have made Earth their next target for conquest — and the aliens are already among us.

Two nuclear warheads have been stolen; there are traitors to the human species in the highest ranks of the army; and alien infiltrators have assumed human form. Only one person seems to know what's going on: the army's mysterious scientific advisor, the enigmatic Major Kreer.