Weeks before D-Day, MI5 interrogated a school headmaster whose Daily Telegraph crosswords contained the words Omaha, Utah and Mulberry, three of the most classified code names of the war. It was pure coincidence, but one of those words, Mulberry, is the subject of this episode: the audacious plan to build two complete deep-water harbours in Britain and tow them across the English Channel to Normandy. Because the Dieppe raid had proven that capturing a fortified French port was impossible, the Allies decided to bring their own.

We break down the strange vocabulary of the project, from the sunken Gooseberry blockships and 6,000-ton concrete Phoenix caissons to the wave-breaking Bombardons, the tide-riding Spud pierheads and the flexible Whale roadways. We cover the stealth hydrographic surveys off enemy beaches, the Scottish storm that decide ...