[1] It is the nineteenth in his series of novels featuring Chief Inspector French of Scotland Yard, a prominent investigator of the Golden Age of Detective Fiction.

On a train to Calais completing the final leg of a foreign tour, one of the passengers approaches Harry Morrison, the employee of the travel agency leading the party.

He has an idea to create a company that provides cruise ship tours around the British Isles aimed at passengers on lower incomes who cannot afford expensive foreign travel.

The crucial ingredient is the addition of gambling aboard, with a casino that can operate as the ship cruises just outside the three mile limit and therefore beyond the jurisdiction of British authorities.

Concerns about the damage the ship is doing to Britain's international reputation leads the Prime Minister and Home Secretary to approach the police to take action.