Mysterious Intruder (1946), the fifth entry in Columbia Pictures’ highly successful Whistler franchise, would be the last film of that series directed by William Castle, who had kicked off the first of eight Whistler B-films with The Whistler (1944). During his sojourn at the studio, Castle would alternate between Whistler movies and Crime Doctor vehicles, often directing some of the most entertaining and innovative entries in each series…before leaving in 1947 to venture out as an independent director-producer. Though he did return to Columbia for a brief period from 1953 to 1956, Castle is best known to film fans for his gimmicky horror films that still entertain audiences today, from House on Haunted Hill (1958) to The Tingler (1959) to Homicidal (1961). He was also the producer of the 1968 horror film classic Rosemary’s Baby.

Intruder finds series star Richard Dix playing a private-eye named Don Gale this time around. An elderly music shop owner named Edward Stillwell (Paul E. Burns) stops by Gale’s office after hours to enlist the detective’s help in locating a missing girl named Elora Lund. Many years ago, when Elora was only fourteen, her mother left some knick-knacks and other odds ‘n’ ends with Stillwell to sell…but the old man was too kind-hearted to part with the possessions. Among Mrs. Lund’s effects is a treasure so priceless that Gale agrees to take the case even though Stillwell can only pay him a $100. Gale is assured that, once located, Elora will be able to reward him far more handsomely.

Three nights later, a young woman (Helen Mowery) pays Stillwell a visit and claims to be Elora. Ecstatic that she’s returned, Stillwell calls Gale to let him know…but, when he returns to where he left the woman, he finds himself up against a goon named Harry Pontos (Mike Mazurki), who croaks the old man and kidnaps Miss Lund. But “Lund” is really a woman named Freda Hanson, planted by Gale to find out just what Stillwell has that’s so valuable. Don soon learns that the real Elora (Pamela Blake) stands to inherit a pair of rare recordings from “The Swedish Nightingale” herself, Jenny Lind…worth $200,000.

Many fans of the Whistler movies consider Mysterious Intruder to be one of the best in the series. While I’d still argue that it’s a toss-up between the first Whistler and The Mark of the Whistler (1944), Intruder does have an intriguing plot and, of course, that surprising twist ending that we’ve now come to expect from the franchise. Dix has one of his best roles as a seedy shamus who’d probably sell his grandmother for change, and he’s working with a better-than-average cast of great character actors. The film is, in many ways, a bargain basement version of noirs like The Maltese Falcon — with Dix the somewhat unethical private eye, Nina Vale his loyal secretary, Mowery the mystery woman who’s not what she appears to be, and Regis Toomey as the manager of the apartment building in which Mowery lives…and who can’t be eliminated from the list of suspects, either.

Barton MacLane, who appeared in Falcon as the hard-nosed Lt. Dundy, is on hand here as one of the detectives investigating the case. He played both cops and bad guys in many of the Warner Bros. films of the 1930s and 1940s and is probably recognizable for his roles on TV series like Outlaws and I Dream of Jeannie. He’s partnered with another TV vet, Charles Lane (The Lucy Show, Petticoat Junction), who plays against type here (Lane usually played weaselly bureaucrats and other officious roles). Mike Mazurki, who was just starting to attract notice in films like Murder, My Sweet (1944), plays the doomed Pontos and gives Intruder solid noir credibility. Other familiar faces in the film include Stanley Blystone, Edith Evanson, Kathleen Howard, Arthur Space…and as always, Otto Forrest as the omnipresent narrator.

Mysterious Intruder will air this September 22nd at 10:45am on Turner Classic Movies as part of the channel’s month-long scheduling of these classic programmers based on the popular radio program. Next week at Radio Spirits: The Secret of the Whistler!