Sabotage
From IMDb : Mr. Verloc, a cinema owner, is part of a gang of saboteurs in London. He lives with his wife, Winnie, and her young brother, Stevie. They know nothing about Verloc's secret. Scotland Yard assigns an undercover detective, Ted, to work in a shop near the cinema and investigate the man. The head of the gang assigns Verloc to put a bomb in the metro. The man sends Stevie there with the "bag". Stars: Sylvia Sidney, Oskar Homolka, Desmond Tester, and John Loder