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.

[2] 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.

Carruthers and Davies go on, amidst cryptic warnings-off from circling German naval officers, sailing expeditions among the Frisian isles and inlets, and fights, to carry out covert surveillance at the estuary in question, to discover that the German Empire is using a naval base hidden in the islands to carry out rehearsals for a seaborne passage across the North Sea of a German army with the aim of militarily invading Britain, and that Herr "Dollmann" is in fact Lieutenant Thomas, an embittered former Royal Navy officer who is treasonously assisting their preparations with his detailed knowledge of the British coast and key naval defences.

After sabotaging one of the rehearsals, whilst escaping to the Netherlands by sea in two roped yachts with the information about it, along with a badly wounded Dollmann and his family as prisoners, Davies abandons Dollmann with his wife in the Dulcibella to allow him to return to Germany to seek medical attention for his wounds at the insistence of Clara, who agrees to accompany Davies and Carruthers back to Britain in the Medusa with his papers revealing the German plans in detail.

Dollmann and his wife are murdered by the pursuing German authorities — led by Kaiser Wilhelm II, in person — when the Dulcibella is rammed and destroyed.

"[3] Among the changes were the inclusion of the Kaiser (although his presence at the trial towing of the lighter is hinted at in the book) and the fate of the character of Dollman (in the original novel he drowned himself; in the film he is mortally wounded after being shot, then killed when the Germans ram his yacht.)

[9] The critic from the Observer called the film "an affectionate, commendably straight adaptation... the excitement somewhat abates in the perfunctorily handled scenes ashore... the cinematographer Christopher Challis uses the Panavision screen to fine dramatic effect.