Vallo Castle, German-occupied Denmark, 1943: Jews are being smuggled out of the country on fishing vessels bound for Sweden before Nazi search teams can find them. Gunter Herz, an Austrian-Jewish refugee drafted into the British SOE for 'special duties' is parachuted in, tasked with locating a missing British agent being hidden by Danish resistance. On finding the agent, they are to infiltrate a German U-boat on its way to Ireland, carrying secret scientific technology. But can Gunter get safely out of Denmark without his real identity being revealed? When Gunter goes missing, SIS operative Katharine Simmons is drafted in to locate the U-boat before it is blown out of the water by the Russians, thus losing its precious cargo. But Katharine has made her own jaw-dropping discovery, which will put Gunter, and the whole of Britain, in mortal danger.