A visit to a private London museum takes a sinister turn when fifteen-year-old Sunni Forrest and her friend Blaise are lured through a painted doorway into the past. As a clandestine, eighteenth-century world of art thieves, forgers and bodysnatchers closes in on them, they must take their chances on the treacherous streets of the city. Can they evade their captors and keep an ancient secret from falling into the wrong hands? And will they find the key to unlock the only way home - before it closes forever?