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Dark both visually and in a narrative sense, the latest installment in the Harry Potter series continues the franchise’s transformation from children’s movie to much heavier fare—in this case with a vengeance, some would say. “Harry Potter and the Deathly Hallows: Part I,” the penultimate chapter in the saga, is a grim and scary picture in which humor is in very short supply, shot by director David Yates and cinematographer Eduardo Serra in a dank and gloomy style in which it’s hard to discern a single moment of light amidst all the shadows.

Of course that’s reflective of J.K. Rowling’s book (or rather the first half of it), adapted by Steve Kloves, which is essentially a long chase in which chums Harry (Daniel Radcliffe), Ron (Rupert Grint) and Hermione (Emma Watson) are pursued by the ruthless minions of malevolent Lord Voldemort (Ralph Fiennes) while themselves trying to hunt down the Horcruxes—items containing portions of Voldemort’s soul—that he needs to complete his resurrection. In essence that’s the entire narrative, and since it ends inconclusively, with a cliffhanger that portrays the—presumably temporarily—triumphant villain clutching the ultimate object of the youngsters’ search, even after nearly two-and-a-half hours you’re likely to leave it feeling a mite shortchanged.

The plot does, however, boast a chain of episodes in which the trio of heroes manage to find some of the prizes they’re searching for while escaping the clutches of Voldemort’s supporters again and again by the slightest of margins. And in one of them—a prolonged shape-shifting sequence in which, disguised as a threesome of grown-up bureaucrats, they snatch a Horcrux from wicked Dolores Umbridge (Imelda Staunton), the new mistress of the Ministry of Magic—Kloves and Yates actually manage to insert some clever physical fun into the mix.

But that doesn’t last long. Soon the action turns to an extended scene in which Harry, Ron and Hermione hide in a forest, and the evil Horcrux begins to take a toll on their friendship. The somber tone of this episode ratchets up the sense of desperation even in a twist that should involve exaltation, when Harry locates a sword that’s a formidable weapon. And there’s still further danger involving a few other returning baddies, Bellatrix Lestrange (Helena Bonham Carter), Wormtail (Timothy Spall) and Lucius Malfoy (Jason Isaacs)—that ends in the youngsters’ losing a friend, an event that adds to the pervasive sense of gloom.

And this doesn’t even begin to cover all the old characters and new ones that amble in and out of the action, mostly for brief, and not often very happy, appearances. Those unfamiliar with the “story so far” are advised that despite some flashbacks there’s little effort to bring them up to speed, and they’re likely to feel as lost as our heroes in the forest. But of course this picture isn’t made for them; it’s intended for the hard-core faithful, and from their perspective its bleakness will represent the darkness before the dawn, the beginning of the summing-up they’ve all been waiting for.

On that level it will certainly serve. Though far from the best installment of the series—that accolade is still shared by Alfonso Cuaron’s “Prisoner of Azkaban” and Mike Newell’s “Goblet of Fire”—it’s far from the worst, Chris Columbus’ two initial features. By now Yates has developed so sure a sense of the material that he can feel free to indulge in a bit of inspired innovation—an animated sequence that tells, to Rhys Ifans’s narration, the story of the three brothers that explains the “Deadly Hallows,” and adds a touch of magic to the picture that even the best CGI (which includes the re-appearance of elf Dobby, voiced by Toby Jones) can’t quite match.

The starring threesome have grown into the roles over the years as well, even if this time around they’re somewhat straightjacketed by the seriousness of the characters’ plight. Radcliffe is barely given an instant of escape from his clenched-teeth expression, and Watson is similarly limited to a generalized concern, though she does have a couple of angry outbursts to toy with. As usual, the opportunity for levity falls mostly on Grint, who can draw a smile even in the most emotional situations. Of the rest, Fiennes has his big moments at the beginning and close, but they’re abbreviated, as are Staunton’s scenes as Umbridge; and old friends like Snape (Alan Rickman), Hagrid (Robbie Coltrane), Moody (Brendan Gleeson) and Lupin (David Thewlis) appear only fleetingly. Providing some compensation are newcomers Bill Nighy, as the officious but short-lived Minister of Magic, and Ifans, as oddball recluse Xenophilus Lovegood.

At this stage one can take for granted the elegance of the film, to which production designer Stuart Craig, the team of art directors supervised by Neil Lamont, and costume designer Jany Temime all contribute substantially. And while one might consider the visual palette Yates has chosen excessively dark, Serra certainly does a masterful job at presenting it. Another newcomer of consequence is composer Alexandre Desplat, who contributes a score that’s both effective and refined, though not terribly memorable.

For committed fans of the series, this initial part of “Deathly Hallows” will certainly be a must, and won’t disappoint. For those whose degree of devotion to the previous installments has been more lax, some boning-up might be in order before taking it in. For newcomers, though, it’s definitely not the place to start.