Despite little evidence of their activities, the British public became obsessed with the idea of German spies lurking around every corner during the war. Fuelled by popular fiction and tabloid newspapers, this paranoia overestimated both the number and the quality of enemy spies on British soil, laying blame on those damned Krauts for any and all misfortune.
Nevertheless, espionage was a real threat in every country involved in the war, with military codes and advanced notice of troop movements at risk of falling into enemy hands. In Britain, it was up to Scotland Yard and MI5 to separate the imagined spies from the real ones, the most famous of which was Carl Hans Lody. With very little training our guile, Lody was discovered sending telegrams to Germany using simple codes and at times even plain text. A real genius move, that. He was arrested in Ireland in October 1914 and executing in the Tower of London the following month. He was the first person to be executed there in 167 years, so it's not like he never achieved anything. Ten other spies would follow his fate in the remaining years of the war, with others imprisoned or executed elsewhere.