The book begins on board the sinking ship. Knowing that women and children are more likely to escape in lifeboats, a mysterious American man entrusts vital government documents to a “patriotic” young American named Jane Finn. It is a draft treaty, which promises to compromise the British government if it falls into the wrong hands. The rest of the novel sees the amateur detectives, Tommy and Tuppence, risk life and limb to recover the missing woman and the papers they hope are still in her possession.
Although much of the book is set in London, the reader’s attention is repeatedly pulled back to Irish Sea crossings. Fortunately surviving the U-boat attack, Jane Finn is taken to Ireland along with other survivors, and thence re-crosses the channel to Holyhead. Likewise, a false clue leads the detectives to search for the missing papers at that Welsh port town.
The Irish Sea ripples threateningly underneath the rest of the novel, regardless of where the action is taking place. Crossings have risked the characters’ lives and Britain’s national stability, and Tommy and Tuppence must return repeatedly to the history of the Lusitania while the security of the latter remains in doubt.