London 1930s: Verloc, the German manager of a cinema, is an undercover saboteur with an anarchist cell. Their leader is unhappy with attempts to disrupt the city so witholds payment until Verloc delivers a proper strike: bombing Piccadilly Circus underground.
Verloc's unhappy American wife Sylvia and her younger, clumsy brother Stevie are befriended by Ted Spencer from the neighbouring shop, unaware that he is a detective undercover observing the under-suspicion Verloc.
Velroc discovers Ted's identity the night before the attack so, to escape suspicion, gives Stevie the bomb to leave in Piccadilly which leads to the famously tense sequence as Stevie is delayed on his journey and is killed when the bomb explodes on a bus.
Hitchcock's taut direction elicits nervy performances from Sylvia Sidney and Oscar Homolka and a characterful supporting cast.
Shelf or charity shop? It's ticking away on my shelf...