Poet Siegfried Sassoon survived the horrors of fighting in the First World War and was decorated for his bravery, but became a vocal critic of the government's continuation of the war when he returned from service. Adored by members of the aristocracy as well as stars of London's literary and stage world, he embarked on affairs with several men as he attempted to come to terms with his homosexuality.
During a handover to the head of counter-terrorism of MI5, Harry Pearce, a terrorist escapes custody. When Harry disappears soon after, his protégé is tasked with finding out what happened as an impending attack on London looms, and eventually uncovers a deadly conspiracy.