In the summer of 1940, Britain's survival depends partly on a technology the Luftwaffe does not know exists. This History Hit documentary traces radar from Robert Watson-Watt's early failed attempt at building a death ray to the towering Chain Home stations strung along the English coast, which gave RAF fighter controllers advance warning of incoming German raids during the Battle of Britain. The film follows the scientists and engineers who turned a rejected weapons concept into a detection system, and shows how the same technology was adapted for use at sea, helping convoys and escort ships hunt U-boats during the Battle of the Atlantic. Archival footage of Chain Home masts, RAF operations rooms, and period interviews sit alongside expert narration explaining the physics in plain terms. The story treats radar as a hidden hinge of the war, a piece of infrastructure invisible to the enemy that shaped the outcome of two separate campaigns fought years apart.