Dorothy O’Grady is uniquely placed in the annals of espionage. She was the first Briton condemned to death under the Treachery Act of 1940 after she was frequently spotted on the outskirts of Sandown (a prohibited area on the Isle of Wight), insisting time and again that her dog had strayed. Had her appeal not saved her from the gallows, she would have been the only woman of any nationality to suffer death under the Act during the Second World War – indeed, the only woman to be executed in Britain for spying in the 20th century. Yet the full story of her extraordinary brush with notoriety and its enduring legacy has never been told, despite the fact that it has more than once dominated the front pages of the British press and inspired both a BBC radio drama and a novel. Now, with the benefit of access to previously classified documents, the truth underpinning the O’Grady legend can finally be revealed. Following her appeal she served nine years in prison for her wartime crimes – but was she really a spy in the employ of Germany? Or was O'Grady, as she insisted years later, a self-seeking tease who committed her apparent treachery ‘for a giggle’? Or was there some other motivation which drove her to wartime infamy in a case which reverberated around the world? In The Spy Beside the Sea, author and journalist Adrian Searle examines all the evidence to reach a disturbing conclusion.