Christmas, 1940. A temporary truce between the German and Allied forces is a welcome respite from the relentless air raids over London. Down Street underground station, in the heart of Mayfair, is now a secret retreat for Prime Minister Winston Churchill and his cabinet.

In this supposedly secure location, the body of a woman is found, stabbed in the heart. Detective Chief Inspector Coburg and Sergeant Lampson are called to investigate.

However, whispers of treason as well as the suspicion of insidious Russian plots muddy the waters of the case, and personal resentments strike far too close to home. Everything is on the line for Coburg and Lampson as the body count steadily rises.