Before the war, Lord James Lord Horn and Anthony Hillman had been the most celebrated private investigators in London since Sherlock Holms. With WW1 raging, they have retuned to the army to head up MI-1, the counterintelligence Bureau for the UK's domestic security.

Meanwhile, Anders and Knut Holmson, two Norwegian brothers, German spies called the Ravens, have smuggled a dirty old steamer trunk into Ireland, hiding within is a secret super-weapon destined for London. That trunk's arrival signals the activation of the Swallows, Annalee and Tamera Gebhardt, German sleeper agents hiding out in the open as members of the British aristocracy. They have waited for over two decades to be awakened. Their mission is to commit genocide.

It's up to Horn and Hillman to thwart this plot and save England from annihilation.