Mysterious Intruder was the fifth entry in Columbia's B-picture series based on the radio anthology "The Whistler". Richard Dix, the leading man in all but one of the "Whistler" films, stars as duplicitous private eye Don Gale. Motivating the storyline are a pair of priceless Jenny Lind wax recordings, which are coveted by a Swedish millionaire. Someone is willing to kill to get his or her hands on the records, prompting Gale and the cops to conduct a citywide search for the killer. The film's resolution is surprising only to those who hadn't seen the previous "Whistler" films, but it still works. Predominant in the supporting cast is Mike Mazurki, offering a virtual reprise of his "Moose Malloy" characterization from Murder My Sweet (1945).