Presenter Jules Hudson follows a deep-water diving team as it searches the seabed off the coast of Northern Ireland for the forgotten shipwrecks of the Battle of the Atlantic, the longest naval campaign of the Second World War. Among the first discoveries are an Allied escort vessel sunk in battle 40 miles off Donegal, and the wrecks of German U-boats that prowled the Northern Ireland coast looking for targets. On dry land, Jules uncovers the remains of a massive US military base in Derry.