4 ideas
In 1880s Melbourne a cabman picks up a drunk man, Oliver Whyte, with a gentleman in a light coat and evening dress. The gentleman leaves, a similarly dressed man later gets in and gets out, and Whyte is found dead from a chloroform-soaked handkerchief. Brian Fitzgerald is arrested because he fits the description, but the killer is Whyte's friend Roger Moreland, who wanted papers exposing the secret first marriage of the millionaire Mark Frettlby.
Witnesses remember a type of person, such as a gentleman in a light coat and evening dress, rather than a face. Any man who fits that type can be matched to the testimony. An investigator who fixes on the first suspect who fits will read every later fact as confirmation, so a class-uniform description becomes evidence against whoever is nearest.
An innocent suspect can refuse to say where he was because the true account would expose someone he loves. Fitzgerald spent the night in the slums with a dying woman who told him a secret that would disgrace his fiancée's family, so he chose to stay under suspicion rather than reveal it. The protective silence of the innocent sustains the case against them as effectively as guilt would.
The reputations of the rich are kept safe by knowledge that sits among the poor. Marriage records, dying confessions and illegitimate children live in the slums and can be carried back into the drawing room as blackmail. Read the city this way and high society and the slums stop looking like separate worlds: they are one economy of secrets, and the slum witness decides the fate of the gentleman.