4 ideas
A cask shipped from France to London is damaged while being unloaded, and a shipping clerk sees gold sovereigns and a woman's hand inside. The cask is claimed by Léon Felix, a London-based Frenchman, and then disappears. The body is identified as Annette Boirac, a Paris industrialist's wife.
An alibi assembled from trains, boats, and carters is a chain of claims about time and place, and each claim can be checked against independent records such as tickets, porters, hotel registers, and shipping manifests. Its strength is that of its least-verifiable leg. An investigator who checks each segment exhaustively will eventually find the one gap where the suspect could have been elsewhere.
When every clue converges smoothly on one obvious suspect, treat the convergence as a possible product of design rather than proof. A clever culprit makes the case against someone else coherent. So the coherence of a case is itself a clue to be tested, especially when the evidence could only have been arranged by someone with prior access and a motive to deflect.
The case is solved by a relay of investigators: a Scotland Yard inspector, a Paris Sûreté detective, and finally a hired private detective. Each proceeds through patient elimination, re-interviewing witnesses and tracing physical objects step by step, rather than through intuitive leaps. Progress comes from exhausting possibilities and handing off accumulated facts, so that dull thoroughness rather than brilliance cracks the case.