A mysterious criminal known only as 'The Shadow' is blackmailing the wealthiest citizens of London. Scotland Yard is on the case, but anyone who discovers clues to the Shadow's true identity is murdered. Chief Inspector Fleming believes that the killer is one of the guests at his sprawling mansion. But with his boarders including a former safecracker and a mystery novelist in search of his next bestseller, uncovering the Shadow's secret may not be as easy as he thinks.

The Shadow is based on a popular stage play that ran for years in London's West End. Star Henry Kendall had previously headlined the stage version. He was also the leading man in the early Alfred Hitchcock film Rich and Strange (1931). Playwright Donald Stuart later emigrated to the United States to become an actor, appearing in classics such as The Invisible Man (1933) and The Canterville Ghost (1944). The villain of The Shadow is not connected to the heroic character of the same name that appeared in pulp magazines starting in 1931 and would be popularized on radio by Orson Welles. However, the fact that both characters share the same costume indicates that the pulps' covers may have had some influence on the filmmakers.