弗里西亚群岛位于荷兰和德国所属的北海沿岸。在群岛附近航行有时会有危险，尤其是在恶劣天气下，因为这里有不少沙洲，涨潮时会隐藏在浅水下面。沙洲间的航道很窄，不易察觉，而沙洲本身也会随着海风和潮汐发生改变。

年轻的阿瑟·戴维斯是个航海爱好者，他热情勇敢、技术娴熟，喜欢驾着他的“达尔茜贝拉”号游艇穿梭于这片复杂、危险的海域。戴维斯邀请他的朋友卡拉瑟斯从伦敦赶来，二人一起出海度假，可在船上待了几天之后，卡拉瑟斯才弄清此行的真正目的。看来，他们是要去解开一个谜，还要解开一个名叫多尔曼的男人身上的疑团。两个好朋友开始了调查——与此同时，他们也引来了一位德国炮艇艇长的密切关注。

故事发生在1902年，因此这段海岸的危险之处不仅仅在于沙洲。第一次世界大战的阴云不断汇聚，年复一年，慢慢地愈来愈暗……

The Frisian Islands lie along the North Sea coast of Holland and Germany, and sailing around these islands can be dangerous, especially in bad weather, because of the sandbanks that lie hidden beneath the shallow waters at high tide. The channels between the sandbanks are narrow and easy to miss, and the sandbanks themselves change with the wind and the tides.

Arthur Davies is young, enthusiastic, and a brave and skilful sailor, who takes great delight in sailing his yacht Dulcibella through these difficult and dangerous waters. He asks his friend Carruthers to come out from London and join him for a sailing holiday, but his reasons for doing this only become clear to Carruthers after several days on board. It seems there is a riddle to solve, and a little mystery about a man called Dollmann. The two friends begin to investigate – and the commander of a German gunboat begins to take a close interest in them.

For the year is 1902, and sandbanks are not the only danger on this coast. The gathering storm-clouds of the First World War are slowly growing darker, year by year...