Humanity's best kept secret may be its best hope for survival. In the midst of World War Two, Enigma codebreaker Alan Turing finds himself on the brink of a discovery that will change the world: a way to send a message back into the recent past. But when his invention successfully averts disaster on D-Day, the danger it poses in the wrong hands becomes terrifyingly clear. As Turing's enemies close in, only one decision is possible: Nautilus must be kept hidden from the world, safe in the care of the only person he can trust. Seventy-five years later, Annabelle McIntosh's life is upended when she learns she will become the latest custodian of the greatest secret of the modern age - Turing's Nautilus machine. Following in her family's footsteps, and bound by Turing's strict principles governing the use of his creation, Annabelle is soon faced with the same impossible choices. Nautilus has the power to prevent catastrophe, but the wrong decision could have devastating consequences. When the stakes are at their highest, will using the machine save the world - or destroy it?