Actor-director Kenneth Branagh has been making solid adaptations of classic Agatha Christie mystery novels since his 2017 take on her classic “Murder on the Orient Express.” His latest adaptation, “A Haunting in Venice,” is the most ambitious yet, turning a methodical whodunit into a horror-mystery hybrid. Problem is, this mysterious “Haunting” isn’t spooky enough for the horror fans, nor fun enough for mystery lovers — and the result is a muddled mess of a film.

By the admission of 20th Century Studios head Steve Asbell, “A Haunting in Venice” is an adaptation of one of Christie’s “lesser-known” novels, “Hallowe’en Party,” and a lot of creative liberties have been taken to transform this “Party” into a “Haunting.” Instead of a party, detective Hercule Poirot (Branagh) now heads to a seance set in the titular Venice, instead of the novel’s party in the British countryside.

Key members of the book’s cast are removed entirely, and the ones that do remain are heavily modified. Joyce Reynolds, a 13-year-old girl in the novel, is now an adult medium played by Oscar winner Michelle Yeoh. Changes when adapting a book for the screen aren’t new, but these changes only exist to serve a story that is an obvious downgrade from its source material.

“A Haunting in Venice” opens on a sequence of establishing shots that look solid but don’t ultimately accomplish much and go on for too long. Even when detective Poirot finally enters the picture, this becomes a theme. Questionable cinematography choices litter the film, from frequent shots of irrelevant objects to muddy-looking night sequences.

The cast populating these shots is one of the better aspects of the film. Branagh may not always be the best director for Poirot stories, but he captures the essence of the snobby, intelligent and very mustachioed detective perfectly. Yeoh steals the show in her limited screen time as Joyce, selling her bizarre character in a way that makes the viewer take her seriously. Another standout is Jamie Dornan, playing anguished doctor Leslie Ferrier with jarringly effective weight.

Unfortunately, not all stars align for this cast. Tina Fey portrays mystery author Ariadne Oliver, and her character is heavily downgraded from her status in the novel as a sort of co-lead for Poirot. In the film, she serves exclusively as an exposition deliverywoman, and Fey’s flat performance does not help to punch up this characterization.

That brings us to the most lackluster part of the film: the script. Screenwriter Michael Green brings the audience to a story that is entirely different from the novel in tone and plot, and this is rarely to anyone’s benefit. The original “Hallowe’en Party” is a novel about a girl who says she saw a murder, then gets murdered just hours later. The simple premise that slowly branches out into a more sprawling tale involving a forged will and tensions within a small town. “A Haunting in Venice” cannot be similarly simplified, which does not work in its favor.

Before the first murder is even committed, the script is throwing in ideas about an allegedly haunted house, a splintered family, the supernatural, a broken-off engagement and the anniversary of a daughter’s death. While these elements become part of the mystery eventually, they lead to a narrative that isn’t concise enough to be understandable.

But this isn’t supposed to be just a whodunit mystery — it’s also supposed to be a “supernatural thriller.” As the body count begins to climb and questions of the supernatural become more pronounced, the film leans into the idea that the house could actually be haunted, with shots that will make audiences more unsettled or perplexed than genuinely scared.

By the time Branagh finally decides to resolve its “supernatural” occurrences, detective Poirot’s answer to what caused it all is so disappointing that it’s almost hilarious. Even still, the film closes on a twisted version of “you gotta believe in magic!” This final ambiguity feels like it was intended, but that doesn’t mean it isn’t frustrating to watch in the whodunit genre, which is all about delivering satisfying answers to these kinds of central dilemmas.

It’s not as if “A Haunting in Venice” doesn’t have redeeming qualities. A mostly solid cast delivers passable dialogue over pretty shots of a gothic 1940s Venice, and the attempt to blend a whodunit with a horror flick makes for a combination with a ton of potential. It’s a unique risk and genuinely new idea that audiences should be hoping to succeed in the age of sequels and spin-offs. But the execution has to be there for an idea for this audacious combination to work — and, unfortunately, it isn’t.