Riddle of the Sands — LibriVox Audiobook

This synopsis covers the underlying book linked to this recording: Riddle of the Sands. Recording-specific abridgements, added material or adaptation changes have not been verified.

Carruthers, a minor official in the Foreign Office, is contacted by a former university acquaintance, Davies, inviting him to join in a yachting holiday in the Baltic Sea. Carruthers agrees, as his other plans for the social season have fallen through because of a heartbreak.
He arrives to find that Davies has a small sailing boat (named Dulcibella, a reference to Childers's sister of that name), not the comfortable crewed yacht that he expected. However Carruthers agrees to go on the trip and joins Davies in Flensburg, on the Baltic, whence they head for the Frisian Islands, off the North-Sea coast of Germany. Carruthers has to learn quickly how to sail the small boat.
Davies gradually reveals that he suspects that the Germans are undertaking something sinister in the German Frisian islands. This is based on his belief that he was nearly wrecked by a German yacht luring him into a shoal in rough weather during a previous trip. The yacht was owned and captained by a mysterious German entrepreneur called Dollmann, whom Davies suspects of being in fact an expatriate Englishman posing as a German. The situation was further complicated by Davies having fallen deeply in love with Dollmann's daughter, Clara, who, Davies is sure, is not involved in whatever nefarious scheme her father is engaged upon. In any case Davies is suspicious about what would motivate Dollmann to try to kill him and believes that it is some scheme involving the German Imperial government. Having failed to interest anyone in the British government in the incident, Davies feels it is his patriotic duty to investigate further on his own – hence the invitation to Carruthers.

Carruthers and Davies spend some time exploring the shallow tidal waters of the Frisian Islands, moving closer to the mysterious site where there is a rumoured secret treasure recovery project in progress on the island of Memmert. The two men discover that Dollmann is involved in the recovery project. Carruthers and Davies try to approach Memmert. They are warned away by a German Navy patrol boat, the Blitz, and its commander, von Brüning, who is friendly and affable but still makes a veiled warning. This makes them all the more sure that there is something more than a treasure dig on the island. And meanwhile they discover that not only is Dollmann indeed an Englishman but he had been an officer in the Royal Navy – evidently having had to leave Britain in a hurry and take up a new life as a German.
Taking advantage of a thick fog, Davies navigates them covertly through the complicated sandbanks in a small boat to investigate the Memmert site. Carruthers investigates the island. He overhears von Brüning and Dollmann discussing something more than treasure hunting, including cryptic references to "Chatham", "Seven" and "the tide serving", and hears of a rendezvous at the Frisian railway station, several days ahead. The pair return through the fog to the Dulcibella, moored at the island of Norderney. There they find Dollmann and von Brüning have beaten them and are seemingly suspicious. However getting in the fog from Norderney to Memmert and back is a nearly impossible feat, which only Davies' superb seamanship could have achieved, and the Germans do not seriously suspect them of having done that.
Von Brüning invites them to Dollmann's villa for dinner, where he attempts to subtly cross-examine them to find out if they are British spies. Carruthers plays a dangerous game, admitting they are curious. But he convinces von Brüning that he believes the cover story about treasure and merely wants to see the imaginary "wreck". The party also serves to show that the Germans do not fully trust the renegade Englishman Dollmann and that there is some rift between them that might be widened.
Carruthers announces that the Foreign Office has recalled him to England. He heads off, accompanied part of the way to the Dutch border by one of the German conspirators – outwardly an affable fellow-traveller. But instead of embarking from Amsterdam to England, he doubles back, returning to Germany in time to be present at the conspirators' rendezvous (to which Dollmann, significantly, was not invited). He manages to follow von Brüning and his men without being noticed and trails them to a port where they board a tugboat towing a barge. Carruthers then sneaks aboard and hides, and the convoy heads to sea.
Carruthers finally puts the riddle together. The Germans are linking the canals and the railways, dredging passages through the shifting sands and hiding a fleet of tugs and barges. The only explanation is that they are preparing to transport a powerful German army secretly across the North Sea to invade Britain's east coast. He escapes after grounding the tugboat, rushes back to Davies and explains how they must flee before the Germans come after them. They persuade Dollmann and Clara to come with them to avoid Dollmann's being arrested by the Germans, who will think he has changed sides again. They promise Dollman immunity from being charged for treason in Britain – which, acting on their own and not having any authorization from the British government, they are not in a position to promise. As they sail across the North Sea, Dollmann commits suicide by jumping overboard, presumably to avoid disgrace and probable arrest.
An epilogue by the "editor" examines the details of a report prepared by Dollmann, outlining his plan for the invasion force. A postscript notes that the Royal Navy is finally taking countermeasures to intercept any German invasion fleet and urges haste.