The little gray cells don’t seem to be as impressive as they used to. In “Death on the Nile,” director Kenneth Branagh once again dons the mustache he famously wore when he directed and starred in 2017’s “Murder on the Orient Express,” but the follow-up film seems to forget it’s supposed to be a mystery at times.

I am in favor of a film genre that sports huge, all-star casts taking on an Agatha Christie classic, but “Death on the Nile” falls a little short of the benchmark. For one, I’m not sure if we needed a “dark and gritty” Hercule Poirot, but it seems like director and star Branagh, along with screenwriter Michael Green, thought so.

Both are back for a fine follow-up to their 2017 Christie adaptation, this time with a more tragic backstory (besides, you know, the murder) and an attempt to make the “world’s greatest detective” more of an action hero than the literary original. And it turns out basically fine, though nothing nearly as exciting or entertaining as it could or should have been.

The plot follows the same through lines as the novel — meaning fans of the book will already know whodunnit — but it does update the characters, changing a few around, melding them or swapping them out for someone familiar.

Forget the ending to “Orient Express,” too, which saw Poirot (Branagh) swept away to investigate a murder in Egypt. “Death on the Nile” ignores that little nod and instead kicks off with an overlong black and white flashback sequence of the Belgian (and de-aged Branagh) as a young man fighting in the trenches during World War I under the command of a man sporting the same soon-to-be iconic mustache. Through this flashback, we learn that Poirot also had a fiancée, who loved him despite an injury he sustained on said battlefield.

Flashing forward to 1937 London, Poirot is a beloved detective visiting a blues-jazz bar, where along with fussing over the petit desserts he’s offered, he observes a vivacious young couple dancing erotically (in possibly the most awkwardly long and overtly sexual choreography put to screen) to the bluesy sounds of American chanteuse Salome Otterbourne (Sophie Okonedo). The couple, Jacqueline de Bellefort and Simon Doyle (Emma Mackey and Armie Hammer) are recently engaged and infatuated with each other. That is, until Linnet Ridgeway (Gal Gadot), a wealthy heiress and friend of Jacqueline, enters the room.

Flashing forward six weeks, we see Poirot enjoying a holiday at the Pyramids at Giza when he bumps into his old friend Bouc (Tom Bateman) flying a kite from the side of the Great Pyramid itself. Bouc is in Egypt with his wealthy mother, Euphemia (Annette Bening), to celebrate the nuptials of a friend of his, and it turns out it’s Linnet who has married Simon. When Poirot joins the festivities, we learn that the jilted Jacqueline has been stalking the happy couple through their tour of the northern African country, and they are both in fear for their lives over what she might do.

With the pieces set and threats in the air, the group full of suspects, in true Christie fashion, voyages by boat. But not all of them will make it back to port alive, and it’s up to our faithful detective and his little gray cells to solve it.

The film takes a decisive turn from its cozy ’30s mystery roots and opts for this more dramatic turn of the pompous detective. Gone are the quiet, reserved and calculated motions of the little sleuth. Branagh gives us more of a guy who stumbles into the truth rather than deducing it many times.

Though Branagh makes a fine Poirot, he leans into the more serious side of him, faltering slightly when his jovial side should be more prominent. It feels like they’re trying to make Poirot more of an action hero than a simple, brilliant detective, and opting for more melodrama between characters than a more compelling mystery to solve.

Indeed, “Death on the Nile” forgets it’s a mystery at times, with the focus being pulled to the beautiful faces of the supporting cast instead of the world’s greatest detective solving the tangled and sordid story of obsession, love and loss.

Controversies of cast members aside, it’s hard to feel sympathy for anyone except perhaps Bateman, who plays the fun-loving playboy with an infectious spirit, and the addition of beloved British comedic duo from Jennifer Saunders and Dawn French as socialist Marie Van Schuyler and her live-in nurse, Bowers (though they’re not playing for laughs here). Beyond that, most performances are either too bland, too wooden or not fleshed out fully, even by a veteran like Bening.

That’s not to say it’s entirely bad. The genre itself makes up for a lot, and I even enjoyed many moments of it that come later on in the film. There’s also some clever camera work going on, but it’s hard to really love something that seems to forget its main character and gives away too much information too soon, making for a less compelling mystery.

Overall, “Death on the Nile” feels more like a three-hour cruise than the lavish ‘30s murder mystery it should have been.

“Death on the Nile”

127 minutes

Rated PG-13 or violence, some bloody images, and sexual material

2.5 stars