A secret, high-technology international agency called SHADO defends Earth from alien invaders.

The Ministry of Time, a newly established government department, is gathering ‘expats’ from across history in an experiment to test the viability of time-travel. Commander Graham Gore (an officer on Sir John Franklin's doomed 1845 Arctic expedition) is one such figure rescued from certain death – alongside an army captain from the fields of the Somme, a plague victim from the 1600s, a widow from revolutionary France, and a soldier from the seventeenth century.