In the long and spotty history of movie taglines, there have been few quite as noncommittal as the one dreamed up for “Inferno,” the third in director Ron Howard’s series of schlockbusters drawn from the nominal literary oeuvre of Dan Brown. “‘The Da Vinci Code’ and ‘Angels & Demons’ were just the beginning,” proclaim the posters — pretty inarguably, since “Inferno” is nothing if not a continuation of what they started. But there’s a hint of threat in those words too: If you found the first two films soulless and joyless, they imply, prepare for things to have gotten a little bit worse this time. And so it largely proves in the latest installment of perennially endangered symbologist Robert Langdon’s cryptic-lite adventures. As the addled professor dashes around Europe trying to prevent a humanity-culling plague cooked up by a Dante-spouting madman, the film more or less goes through the popcorn motions, but with less technical finesse (and even less mischievous irony) than one might expect from the Howard imprint.

It’s left to a refreshingly diverse international cast of consummate professionals — led, once more, by an increasingly disconsolate-looking Tom Hanks — to breathe what conviction they can into this hoary material, but the result still gives the lie to the old industry maxim that great cinema can spring from trash literature. Sometimes film and novel can be alike in half-heartedly following a template; perhaps the most glowing thing that can be said about “Inferno” is that reliable screenwriter David Koepp (returning from 2009’s “Angels & Demons”) has fully captured the essence of its source. Brown acolytes and adult audiences starved for non-supernatural genre fare might respond in sufficient numbers to greenlight another jaw-clenched jaunt for Langdon, though it has been 10 years since “The Da Vinci Code” thudded onto screens — and seven since its follow-up suffered a notable dip in box office.

Already a long time in Hollywood years, that gap feels, if anything, even longer as “Inferno” gradually — very gradually — gets into gear. Now we’re past its pop-cultural zenith, Brown’s brand of cod-educational, cloak-and-dagger storytelling feels slightly dated, while Koepp’s script shows some self-awareness in this regard: “That’s quaint, I use Google,” responds early-millennial doctor Sienna Brooks (Felicity Jones), when Langdon cites a particular reference book. Howard, for his part, appears to have shot proceedings through a slightly yellowed 1990s filter: The tone and aesthetic here often recall that era’s odd spate of gaudily portentous, pseudo-theological thrillers in the vein of “End of Days” and “Stigmata.”

Howard kicks things off, however, with a more incongruous “Vertigo” reference — one of a couple, in fact — as crazed billionaire geneticist Bertrand Zobist (Ben Foster) falls to his death from the top of a bell tower in Florence. We already know he’s a deranged megalomaniac: A pre-credits sequence flashes through YouTube footage of one of his public addresses on the evils of over-population, in which he none-too-encouragingly advises that “pain can save us.” (In case viewers need additional visual shorthand, he’s also played by a sharply bearded Foster in his signature mode of unblinking intensity.) Days later, across town, Langdon is admitted to hospital with an apparent gunshot wound to the head; when he comes to, he has no recollection of how he came to be in Florence at all, let alone with the Carabinieri seemingly out for his blood.

In place of these more useful reserves of memory, however, he’s tormented by copious gungy CGI hallucinations rooted in the eponymous first part of Dante’s “Divine Comedy” — rendered by Howard, d.p. Salvatore Totino and the effects team as a glossy, bile-hued disco Inferno in which assorted demonic beings gather to gurn, baby, gurn. “I’m having visions!” he pleads to assigned medic Brooks, who whisks him away from hospital when his pursuers arrive; “It’s the head trauma,” she replies helpfully. “Inferno’s” lurching, character-heavy plot is rife with supposedly brilliant minds being under-tested in this manner. Once more, Langdon’s reputedly unrivaled puzzle-solving skills are called upon as he traces the circumstances behind his peril, though they don’t get more challenging than picking and arranging letters out of a modified print of Boticelli’s “Map of Hell” — a game of Florentine Wheel of Fortune, if you will.

Such clues wind up leading Langdon and Brooks on the posthumous trail of Zobist, which threatens to end in the dead man’s rather drastic solution to the global population crisis: a global plague of advanced design and Medieval proportions. As this apocalyptic treasure hunt leads them from Italy to Switzerland to Turkey, assorted parties of ambiguous allegiance join the race: World Health Organization director Elizabeth Sinskey (Sidse Babett Knudsen), who has some manner of history with Langdon, her French consort Christoph (Omar Sy) and the enigmatic Provost (a ripe Irrfan Khan, having the most fun of anyone here), head of a shady consulting group on no one’s exact side. The ensuing tangle of crossings and double-crossings is convoluted but not exactly complicated, while there’s a stern, let’s-get-to-work air to the film’s craft and conception that hampers whatever thrill of the chase “Inferno” has to offer. Fundamentally silly the film may be, but it never graduates to spryness.

It says a lot about the multiple blank spaces in Brown’s conception of Robert Langdon that Hanks, the ultra-genial Jimmy Stewart of our day, hasn’t managed in three films to make him any less of a stiff; even with the fate of humanity at stake, it’s hard to work up much emotional investment in this humorless human composite of mansplaining and flannel. Jones, at least, provides some peppery zip to their scenes together, but it’s only the reliably worn-in (and currently, happily, ever-present) Knudsen who projects the blueprint here of an actual person. Her quiet exasperation and steely smarts might even mildly sweeten the prospect of a fourth film in this fusty franchise — that tagline may claim the first two films were the beginning, but promises nothing about “Inferno” being the end. “I need better from everyone! Better!” Knudsen barks at her orderlies in one scene of lukewarm pursuit. You heard the lady, folks.