The Mystery of a Hansom Cab opens with the kind of crime that instantly creates a citywide obsession: a drunken man enters a cab with a companion and arrives dead, chloroformed behind the cab's closed doors. The victim is Oliver Whyte, an unpopular fortune hunter who has publicly quarreled with Brian Fitzgerald over Madge Frettlby. Detective Gorby assembles a powerful circumstantial case against Brian, but Brian refuses to disclose the alibi that could save him because it would expose a woman's secret.
The novel is structurally adventurous. It moves from newspaper report and inquest to police investigation, trial, acquittal, renewed detective work, and finally a family confession. Hume also makes Melbourne itself part of the mystery, moving between St Kilda villas, clubs, railway lines, lawyers' offices, and the Little Bourke Street slums. The contrast is sometimes vivid and sometimes burdened by sensational class prejudice.
The final solution separates apparent motive from actual killer. Mark Frettlby's concealed first marriage gives him the strongest reason to retrieve Whyte's document, but Roger Moreland commits the murder to obtain the same certificate for blackmail. The book remains compelling because each solution creates a moral problem: saving Brian requires exposing another person, solving Whyte's death threatens Madge, and legal truth is ultimately suppressed to preserve reputation.
