After a year of marriage, Edward "Teddy" Bare (Dirk Bogarde) kills his wealthy older wife, Monica (Mona Washbourne), after she asks her lawyer, Phillip Mortimer (Robert Flemyng), to change her will. He stages it to look as if she was accidentally asphyxiated while drunkenly trying to light a gas heater.
To his chagrin, he discovers that she actually intended to leave him all her money; instead, he only inherits the mansion from a prior will, while her millions are left in trust to her only relative, her sister Dora. Edward will receive the money if Dora dies. An inquest rules it an accident, but Phillip makes it clear he suspects Edward. When Edward asks where Dora lives, Phillip tells him she is too far away, in Jamaica.
Edward manages to marry lower-class but well-off widow Freda Jeffries (Margaret Lockwood), who is closer to her husband's age, and much less trusting than her predecessor, keeping tight control over her fortune. As the death of a second spouse so soon after the first would be highly suspicious, he is powerless to do anything. Edward becomes acquainted with Charlotte Young (Kay Walsh), who is looking for a house to purchase for an equestrian school. As Edward was an estate agent before he married Monica, he shows her around, making Freda jealous.
Edward lures Charlotte to his mansion late one night while Freda and the servant are out. He reveals he knows that Charlotte is actually Dora. Then he brazenly admits killing her sister before trying to make her leave. Suspicious, she remains where she is. Eventually, however, Freda returns home and escorts Charlotte to the door. After she drives away, Edward tells Freda that he killed Monica, secure in the knowledge that a wife cannot be made to testify against her husband, and that he expects to inherit Charlotte's money shortly, as he has tampered with the brakes on her car. He is shocked when Phillip enters the room, having heard his confession, followed by his intended victim. She had returned to the house after meeting the lawyer at the estate's gate. Edward flees in his car, but the entrance is blocked by Charlotte's and Phillip's automobiles. With Phillip in pursuit, Edward switches to another vehicle, only to realize too late that he has taken Charlotte's. He loses control and drives off a cliff.Dirk Bogarde as Edward "Teddy" Bare
Margaret Lockwood as Freda Jeffries
Kay Walsh as Charlotte Young (in reality Dora, Monica's sister)
Kathleen Harrison as Emmie, the Bares' servant
Robert Flemyng as Phillip Mortimer
Mona Washbourne as Monica "Moni" Bare
Philip Stainton as Charlie Mann, a business associate of Edward
Walter Hudd as Coroner
Lita Roza as Singer
The film was based on the play Murder Mistaken by Janet Green. Green wanted Dirk Bogarde to be in the play but he turned it down and Derek Farr played the role instead. When Lewis Gilbert was making The Sea Shall Not Have Them he saw the play and thought it would make a good film, and he persuaded Bogarde to play the lead.
Bogarde persuaded Margaret Lockwood to co star. "I was dubious about being able to lay such a character, though I liked her honesty," said Lockwood.
"I think it was a very interesting plot, very claustrophobic," said Gilbert. "I think it was the best thing Margaret Lockwood did, she was great in the film."
Dirk Bogarde later said "the unwholesomeness of the hero was what was fun about it."
Bosley Crowther of The New York Times wrote that the actors are skilled but "they are not offered many opportunities to make Cast a Dark Shadow mysterious or tense."
Lewis Gilbert later said "it was reasonably successful but by then Margaret [Lockwood] had been in several really bad films and her name on a picture was rather counter-productive."
Dirk Bogarde said "the film was a failure":
It was the first time I had come under another star's name - Margaret Lockwood - and it just died, which was a pity because it was a very good movie and I had persuaded Maggie to do it. I remember being on tour in Cardiff with a play and I saw a poster for Cast a Dark Shadow and it had 'Dirk Bogarde in Cast a Dark Shadow' and, at the very bottom, 'with Margaret Lockwood'. They altered the billing order because they saw it was dying and that, astoundingly, her name had killed it, though it was probably her best performance ever.
"I'm glad I did it, but am still wondering exactly where it got me," said Lockwood in 1973. After making the movie she did not appear in a feature film for another 21 years.
Cast a Dark Shadow was given a DVD commercial release by Simply Media in June 2015 - nearly 60 years after its theatrical release.