A nebulously sinister title disguises the fact that this is actually a "Boston Blackie" mystery, the seventh in Columbia's series. Reformed criminal Boston Blackie (Chester Morris) is accused of stealing the Niles diamond from a charity function. The police cut a deal with Blackie: If he'll locate the gem, they'll drop the charges. This time the cops go so far as issuing Blackie a police badge, which he uses with amusing abandon. One Mysterious Night, together with The Chance of a Lifetime (43) and The Phantom Thief (46), was given a non-identifiable title so that Columbia could coerce non-"Boston Blackie" fans into the theatre.