The Riddle of the Sands is a 1979 British spy thriller cinema film based upon the novel of the same name written by Erskine Childers. Set in 1901, and starring Michael York, Simon MacCorkindale and Jenny Agutter, it concerns the efforts of two British yachtsmen to avert a plot by the German Empire to launch a seaborne military invasion of the United Kingdom.

Plot

In the Autumn of 1901, Carruthers, an aristocratic junior official in the British Foreign Office, is invited on a yachting and duck-shooting holiday by an old University acquaintance called Arthur Davies. On Carruthers' arrival on Germany's northern coast to join the yacht Dulcibella, Davies explains to him that he has a hidden agenda for the trip and the invitation beyond duck-hunting. While boating around the Frisian Islands ostensibly correcting antiquated British sea charts of the coastline's shifting topography, by chance he had met a retired German sailor called Dollmann on the yacht Medusa with his wife and daughter, Clara, with whom Davies has initiated a romantic attachment. He narrates further that whilst sailing together along the coast in a gale Dollmann had, when Davies had tried to put into a particular estuary for shelter, inexplicably prevented him from entering by executing a deliberately hazardous sea-manoeuvre, to the degree that both their lives had been endangered by it. Davies then reveals to Carruthers that his real interest in the area is that he suspects that the Imperial German Navy is engaged in covert military activity of some nature in the Frisian Islands, with the intention of threatening the security of the North Sea from the British perspective, which the Royal Navy is strategically misdirected to meet, and he is engaged in trying to discover what it is. This the pretext of the "holiday" that he has invited Carruthers upon, given Carruthers' ability to speak German along with his professional contacts within Whitehall, if they should discover something warranting the alarm being raised within the halls of the British Government.