A survey of the defensive (and offensive) work of the RAF, and especially Coastal Command, over the seas around Britain.

Convoy protection (an essential prelude to the invasion of Europe) is illustrated briefly with film from the Atlantic and Mediterranean. A Sunderland of Coastal Command flies over. Captured film: a U-boat torpedoes a cargo vessel, and a Ju 88 bombs a convoy (good shots within the German plane - the pilot looks pleased as survivors struggle onto rafts); Hitler decorates U-boat commanders, and crewmen are also decorated; Admiral Doenitz, with senior colleagues, addresses assembled U-boat crews. But Coastal Command begins to "find and strike" from bases in Britain, Gibraltar and the Azores. A U-boat captain attempts to dive before a Catalina can attack; a Mosquito tackles a Condor (FW 200K); various dogfights are shown. Hitler smiles as the 'Bismarck' is launched, but the battleship at sea is observed by Coastal Command Catalinas - sequence on the sinking (use of 'reconstruction'). As convoys get through, and Britain equips for the invasion, focus shifts to the Channel. The U-Boats' steel and concrete pens are attacked by Bomber Command with 12000 lb bombs; S-boats (shown arming with torpedoes at Le Havre) are also bombed. German observers sound alarm as the invasion approaches; U-boats and S-boats rally. Film of Lancasters, Beaufighters; US troop carrier fighting off German air attack; the beach, after successful landings. Allied land advance forces greater German reliance on coastal shipping - but leaves the RAF also more free to attack. The 'Tirpitz' is bombed and crippled. A Catalina flies into the sunset...