Hitler's Germany made sustained efforts to inflict a terror campaign on the streets of Britain through the use of secret agents and agents provocateurs. The aim was to blow up military, industrial, transport and telecommunication targets, to lower morale among the civilian population and disrupt the war effort. Even before the outbreak of war, the Nazis provided the IRA with assistance for their plan to sabotage the British mainland. Prior to their planned invasion in the summer of 1940, the Nazis were also keen to recruit members of the Welsh and Scottish Nationalist parties to sabotage British targets and, over the course of the war, infiltrated dozens of agents from countries including Norway, Denmark, Holland, France and Cuba. What happened to the myriad plots to blow up Britain? We know that intelligence obtained from decrypted enemy messages via Bletchley Park and double agents like ZIGZAG, SUMMER and TATE alerted MI5 to some of these spies' arrivals, but what about the others? And crucially, was the effort actually half-hearted? Just how well trained were these infiltrators?