4 ideas
In Paris in 1878, Lord Littleby and his servants are killed in his mansion. A golden whale badge clutched in a dead hand is the kind issued to first-class passengers of the maiden-voyage steamer Leviathan, so Commissioner Gauche sails with it to Calcutta and questions the passengers of one first-class dining salon, where more deaths follow before Fandorin exposes the killer.
The story is told in turn through the diaries, letters, and inner voices of different passengers. Each one reads the others through their own class, national, and personal prejudices. The reader has to triangulate the truth from the gaps between these accounts, and the same device lets the culprit's own perspective hide in plain sight.
The suspect who seems fragile, sympathetic, or harmless, such as an expectant young wife, is the one observers instinctively leave off their list. A killer can rely on that social reflex more than on any alibi. Real detection means testing every person against the evidence with equal coldness, whatever role they present.
Gauche and Fandorin work the same closed room with opposite methods. Gauche's methodical routine and his readiness to pin guilt on the most suspicious-looking passengers lead him toward whoever fits his theory. Fandorin keeps observing and testing small inconsistencies without committing early. The contrast shows that an investigator's confidence and seniority say little about accuracy, and that early commitment to a suspect distorts how each new fact gets read.