Sir Maurice Newbury is bereft as his trusty assistant Veronica Hobbes lies dying with a wounded heart. Newbury and Veronica's sister Amelia must take a sleeper train across Europe to St. Petersburg to claim a clockwork heart that Newbury has commissioned from Fabergé to save Veronica from a life trapped in limbo. No sooner do they take off then sinister goings-on start to plague the train, and it is discovered that a mysterious cultist is also on board, determined to recover a precious artefact. Can Newbury and Amelia defeat him before he carries out his terrible revenge? But as Newbury and Amelia hurtle towards the only thing that can save Veronica, they uncover a terrible secret that could threaten the lives of everyone on board. A secret linked to Veronica's past...