In April 1951, the Affray was on a routine peacetime simulated war patrol in the English Channel. She radioed her last position at 21.15hrs on 16 April - and preparing to dive. This was the last signal ever received from the submarine. Fifty-six years later, Alan Gallop uses previously top secret documents, interviews with experts and contemporary news sources to explore how and why Affray became the last British submarine lost at sea.