The Riddle of the Sands recap

Carruthers, a young Foreign Office clerk, accepts his friend Arthur Davies's invitation to join the tiny yacht Dulcibella in the Baltic. Davies reveals that the English-born German resident Dollmann nearly led him onto a deadly Frisian shoal, and he suspects the incident protected a larger secret. The two men survey the tidal channels and pose as harmless sporting sailors while German naval officer von Brüning watches them. Davies's feelings for Dollmann's daughter Clara complicate their work. Carruthers eventually penetrates the restricted activity around Memmert and discovers coordinated preparations for carrying German troops across the North Sea in barges, hidden behind coastal works and local shipping. The pair escape with their evidence and Clara; Dollmann joins them but dies after his treachery against Britain is exposed. Their report alerts British authorities to an invasion plan built on expert use of waters outsiders consider impassable, and Davies and Clara remain together in England.

Carruthers, a young Foreign Office clerk left behind in nearly empty late-summer London, receives an unexpected invitation from his old university acquaintance Arthur Davies to join a yachting trip in the Baltic. Expecting a comfortable cruise, he travels to Flensburg and discovers that Davies's Dulcibella is a very small converted lifeboat with cramped quarters and no crew. Davies himself has become intensely absorbed in seamanship and the German coast. Carruthers initially resents the discomfort and Davies's poor hospitality, but recognizes that his friend is anxious and has summoned him for a reason. As they begin sailing, Davies gradually prepares to explain an earlier incident that changed his plans and made the cruise more than a holiday.

Davies recounts following the yacht Medusa near the Frisian islands. Its owner, Dollmann, an Englishman who has settled in Germany, guided him toward a channel in worsening conditions and then failed to help when the Dulcibella was nearly trapped on the shoals. Davies survived through luck and seamanship. He also met Dollmann's daughter Clara and became deeply attached to her. Studying his log and charts afterward, he concluded that Dollmann's conduct was deliberate and intended to keep him away from something in the islands. Carruthers moves from skepticism to interest. The friends agree to investigate under the cover of an amateur cruise, relying on Davies's sailing knowledge and Carruthers's German and analytical habits.

Carruthers formally commits himself to Davies's inquiry and learns to work aboard the Dulcibella. The men sail through the difficult coastal waters, comparing charts with what they observe and treating navigation itself as evidence. Their survey shows that the channels among the sands are changeable but usable by people with exact local knowledge. They notice signs of organized traffic and guarded activity inconsistent with an empty, harmless coast. Carruthers also understands that Davies's judgment is divided: his suspicions point toward Dollmann, but his love for Clara makes him reluctant to accept the worst interpretation. By patiently mapping approaches rather than forcing a confrontation, they establish the geographical basis of the mystery.

Moving deeper into the Frisian islands, Carruthers and Davies reach Bensersiel and attract official attention. Commander von Brüning, a courteous German naval officer, visits them and demonstrates detailed knowledge of their route. He explains suspicious works and movements as an effort connected with a wreck and possible salvage, while questioning them closely about their intentions. The Englishmen pretend that their interest is merely sporting and commercial, allowing von Brüning to believe he has contained their curiosity. Contact with the Dollmann household brings Davies near Clara again, but it also shows how closely the family is bound to the guarded enterprise. Carruthers concludes that the official explanation is designed to divert them and that continued investigation will require separating their movements and misleading their watchers.

Carruthers and Davies change tactics. While Davies uses the yacht and his attention to Clara to sustain their harmless appearance, Carruthers investigates by land and studies the practical network behind Memmert. A small notebook and the pattern of railways, barges, tugs, stores, and coastal works begin to fit together. The operation is not ordinary salvage: engineers and naval officers are coordinating resources that could move large numbers of men through the shallow channels. Carruthers penetrates the restricted area and observes the central group, including Dollmann, von Brüning, and the engineer Grimm. He now sees that Dollmann's earlier attempt to divert Davies protected a military secret, and that the supposedly useless sands can serve as concealment for a force trained to navigate them.

Carruthers evades the surveillance around Memmert and reunites with Davies, carrying enough evidence to interpret the German preparations. Shallow-draft barges and tugs could assemble troops behind the islands and carry them across the North Sea, while local pilots would overcome the navigational barrier on which Britain might rely. The men plan their escape, but Davies refuses to abandon Clara. Their maneuvering draws Dollmann and his daughter onto the Dulcibella as German authorities close in. Dollmann's English origin and role in a scheme against Britain are exposed; he goes overboard and dies during the flight. Carruthers, Davies, and Clara reach safety with their findings. Back in Britain, Carruthers presents the investigation as a warning that naval defense must account for an invasion launched through the Frisian sands. The secret plan is disrupted, and Davies and Clara are left together in England.

Carruthers and Davies establish that the activity in the Frisian islands is preparation for moving a German army across the North Sea in shallow-draft vessels, using the islands, tidal channels, and coastal railways to conceal mobilization. Their escape becomes entangled with Davies's determination to save Clara from her father's position. Dollmann and Clara leave with them aboard the Dulcibella, but Dollmann, exposed as an Englishman assisting a plan against Britain, goes overboard and dies. Carruthers and Davies bring Clara and their evidence safely home. Carruthers's report warns the British authorities that the German coast dismissed as unnavigable is precisely what makes the scheme dangerous: trained local pilots and coordinated barges could turn it into an invasion route. The immediate plan is compromised, while Davies and Clara are able to begin a life together in England.