The Walkers help Jim Brading moor his sailing cutter Goblin when he misses the buoy. In return he invites them to go sailing aboard Goblin. Their mother agrees provided that they stay within the estuary of the rivers Orwell and Stour and do not pass the Beach End buoy at the mouth of the rivers, and do not 'go out to sea'. These conditions are imposed because of the imminent arrival of the Walkers' father, who, after many years of absence, is expected to return by ferry at any hour from Europe. (It is implied that he has been posted in the Orient.)
The children agree to these conditions. However, on the second morning during a calm, after using the engine for some time, the petrol runs out. Jim, who is serving as captain, rows ashore in the yacht's dinghy from the anchored Goblin to buy more, but does not return, and an unexpected bank of fog drifts over the scene. The Goblin is now without a captain. Some hours later, after hearing the anchor drag in the fog, the Walkers realise that the tide has risen; the anchor chain is now too short and they are drifting. Then, in an attempt to put out more chain, the anchor is lost, and the yacht drifts out beyond Beach End into the North Sea. The children, aboard the drifting boat, decide that it is safer to hoist the sails and go farther out to sea rather than stay near the shore in the sandbanks and shoals of the estuary, which would involve the risk of being wrecked in the fog. Their intention is to put about at daybreak and regain the shore. But as the night continues, the wind rises and it becomes impossible for them to turn around once the fog lifts.
John, who is presented as the most responsible of the children, serves as the 'de facto' captain of the Goblin, but everyone plays an essential role in the adventurous voyage. Running eastward before the wind, the Goblin sails through the night in hazardous conditions. On the following morning, they find themselves approaching the coast of the southern Netherlands. Having little choice but to accept the adventure onto which they have been launched, they pick up a Dutch pilot and arrive safely in Flushing.
The climax of the story has now been reached, for the young Walkers have come to Flushing at the same moment as their long-absent father. John sees the father embarking on a steam ferry bound for Harwich; the father must jump ship to reunite with his family and organize the return voyage aboard Goblin. On returning to the Harwich estuary, the Goblin and its crew are reunited with Jim Brading, who is looking for his missing yacht. In a final point of plot resolution, it is revealed that the absent captain had been unconscious in hospital, suffering from concussion after being involved in a collision with a motor bus.
"This book is Ransome at the top of his form." * OBSERVER * "The book is a record of an uncovenanted voyage, which ended in Holland, of the rain and wind, the darkness and the wild water, the escapes from buoys and from ships crossing in the night, the courage and resource of the children." * EVENING STANDARD * "Perhaps the best of all ... Just what does happen is told with all the wealth of practical detail and satisfying sense of reality which make Mr Ransome so unfailingly successful." * PUNCH * "The most exciting of the whole Swallows and Amazons series." * NEW STATESMAN * "The seventh of the Arthur Ransome books, and I really think it is the best." * SUNDAY TIMES *