Set during the Berlin Airlift of 1948/9 when the Soviet Union blockaded the city's road and rail links, making air transport the only way for the UK and US to supply their sectors of Berlin.
Flight Lieutenant Pat Morgan, one of the many RAF aircrew taking part, becomes friendly with Angela Meier, a young German student helping to unload the cargo at Gatow Airfield.
At her little summer house on the River Havel (which divides East and West Germany) their secret friendship develops into a passionate romance.
But the summer house has another secret purpose ... as a rendezvous point for defectors making the illegal crossing from East to West, swimming across the river at night.
Soon Pat and Angela are caught up in events beyond their control.