This has a darker mood than most Alfred Hitchcock films in his British period. It's a loose adaptation of Joseph Conrad's novel The Secret Agent, which is reconstructed into a series of suspenseful set pieces and opportunities for black comedy.
The content requires a skilled dramatic lead and the production is blessed by the brilliant, liquid eyed Hollywood star Sylvia Sidney. This is nearly as much her picture as a Hitchcock. How frustrating that Robert Donat was cast to co-star but backed out due to illness.
Saboteurs are active around London (their aims are vague) and plan to leave a time bomb at Piccadilly Station which is... entrusted to Sylvia's young brother. He dawdles across London unaware of what his package contains. But we know the time of detonation, and Hitch cuts ever more rapidly between clocks, the package and the fatal distractions of the big city.
The staging and editing of this episode are widely praised, though the Master often used the outcome as an example of bad film making. It is a political thriller, and one of the director's best and most suspenseful thrillers with atmospheric location work and many memorable set pieces.