London, 1851 and all the world has come to Hyde Park for the Great Exhibition. Amidst the splendour of the Crystal Palace sits a black velvet tunnel, guarded by the new police. Within is a diamond as large as a child's fist - the fabled Koh-I-Noor. A custom Chubb lock secures the priceless artefact inside a gilded cage, such that it can be viewed but not touched. And yet... It is a fake. Summoned by Buckingham Palace, he and his partner, Sergeant Wren, are tasked with recovering the true diamond as swiftly and secretly as possible. Pressure mounts as they chase down one false lead after another, gleaning what little they can from the scant evidence, It all ends where it began: in Hyde Park, and the only guarantee is that lives are at risk.