Isaac and Hattie have been invited to a high-profile event at the French National Archives in Paris to finally celebrate their achievement in saving time. But as Isaac prepares to make his speech, the lights go out and a single shot is fired. The police think it was an assassination attempt on the infamous businessman Balthazar Blaise, but Isaac and Hattie realise that something very different is going on when they discover a boy taking something from the archives before vanishing without a trace. As they investigate, Isaac and Hattie are pulled into an underground world, searching secret and forgotten places for a stolen letter, a missing sister and a map that will unearth a long buried secret.