Deep in Scotland Yard in London sits an evidence room, where artefacts of the greatest mysteries in London history are kept. In this 'Black Museum' sits two bullets, fused together after a head-on collision. This was the key piece of evidence in a case that brought together a supernatural Man in Gray and the famous nurse and activist Florence Nightingale - the only person who can see him. Surrounded by war and suffering, the lady enters into a desperate pact with this ghostlike man.