Nancy Brown is an ordinary girl living in the industrial town of Orethorpe at the outbreak of the Second World War. Her life suddenly changes when she is told to report to the mysterious Megg House, and soon she finds herself caught up in a top-secret operation to find German spies operating in the town. Seventy years later Arthur Lane is living in Orethorpe. He strikes up a friendship with an elderly man, Mr Smith. When a newspaper reporter turns up asking to interview Mr Smith about his role in the war, Arthur learns that Mr Smith has a deep secret. Can he discover what it is? The Most Dangerous Spy switches between 1940, when a careless word can leave lives in peril, and 2010, when once-deadly knowledge - including the identity of 'the most dangerous spy of all' - can remain secret no longer...