It's March 1916, and Joseph Reavley, stumbling across the wastes of no man's land to rescue a comrade, is badly wounded. Sent home on sick leave, he finds recovery slow and hard. His sister Hannah is caring for him, but he can't share the grim reality of life in the trenches with her - anxious as she is about Archie, her husband, away at sea. War is changing England at a pace that Hannah finds alarming, and she wonders if the old certainties will ever return. But it's a delight to both her and Joseph when Shanley Corcoran, an old friend, comes to visit. Corcoran confides in Joseph that he's come very close to completing an invention that will paralyse the deadly German U-boats. Soon afterwards, however, disaster strikes. A brilliant young scientist is found murdered, and it's clear that someone has been betraying secrets to the enemy. Joseph's brother Matthew, of the S.I.S., comes down to investigate, and together the two men embark on a search that will solve the crime and lead them to the spy.