General Von Wittke comes to Guernsey, ostensibly to write a report on the Occupation. In fact he has come to see General Laidlaw, a retired British officer and the ex-husband of Von Wittke's sister, a German opera singer. Von Wittke and other generals are aware that Hitler is a lunatic who will ruin the country. They aim to depose him and Von Wittke asks Laidlaw to travel to London to broker a deal. A Gestapo officer arrives on the island, suspicious of Von Wittke's presence, but the general successfully convinces him that he had come to invite Laidlaw to Germany to make anti-British broadcasts and that Laidlaw had refused. The trip to London never happens.
—don @ minifie-1