Another rip-roaring adventure featuring Biggles and his chums Algie, Bertie and Ginger as this time they chase all over the South Indian Ocean to try and track down the German von Schonbeck in his submarine U-517.
The war is over but von Schonbeck has managed to send every ship which he had intercepted to the bottom of the ocean while murdering every man, woman and child aboard in cold blood and into the bargain managing to steal five million pounds worth of gold. His ploy was to stop the ships, order the crew and passengers into the boats, having stated his intention to sink the ship, steal the gold and then turn his U-boat's machine guns on the helpless life-boats.
Air Commodore Raymond not only wants to bring him to justice but also wants the gold recovered so Biggles and his pals set out to try and locate von Schonbeck in the South Indian Ocean. It is no easy task for as Biggles states, 'It is like looking for a needle in a haystack.' The problem is that there are a group of islands, originally discovered in 1762, which very few people have seen let alone landed on. And any one of them might be von Shonbeck's island, where he is hiding the gold and gaining hidden anchorage.
St Paul Island, Prince Edward Island, Macdonald Island, Penguin Island, the Heard Islands and others are considered too inaccessible to be the German's base. But some in the Apostle Group are large enough to be a good retreat and Biggles considers that the most likely to be the one von Shonbeck heads for is Kerguelen Island, which is 45 miles long by eight miles wide. The search is, therefore, focussed on that island.
The problem with Kerguelen is that there is no communication with anywhere and it is 2,100 miles from the nearest inhabited land. But, undaunted Biggles heads that way with two amphibian aircraft. Nearing their destination they spot a Norwegian whaler and some of the sailors on board wave to the passing aircraft but Biggles is suspicious that they are not as friendly as they appear - and as it turns out, he is right to be so.
Finally managing to land on Kerguelen the crew spot signs of recent life and thus begins a great adventure of cat and mouse as each party tries to outfox and shoot up the other. Flitting between Kerguelen and another nearby uninhabited island the tension and excitement mounts and culminates in a dramatic showdown in which lives are lost on the German side but, even though Biggles, Algie, Bertie and Ginger suffer at times and even look as though they might lose their lives, they do not and eventually the quartet turn up triumphant.
Back in London Air Commodore Raymond is exultant and allows Biggles and his men to relax for a short time with Biggles declaring to Raymond, 'As far as we're concerned it's all washed up. Think of something new.' 'I might just be able to do that,' comments Raymond to which Biggles replies, 'But not today I hope?' 'All right - we'll leave it until tomorrow,' says Raymond laughingly and that is exactly what they do!