You must suspend a surprising amount of disbelief while taking in the film “Conclave.”

Based on the 2016 novel of the same name by Robert Harris and in theaters this week, the drama is set within the sacred and secretive Catholic ritual that is the papal conclave, during which cardinals gather at the Vatican to elect a new pope.

Seemingly, screenwriter Peter Straughan and director Edward Berger have stayed fairly faithful to Harris’ tale, which is to say the big-screen version of “Conclave” is chock full of political intrigue, twists and revelations — including a doozy at its conclusion.

It’s simply a bit too much to be taken all that seriously.

Suspend that disbelief, however, and you are treated to a consistently entertaining — and sometimes downright thrilling — affair, one benefiting from terrific performances by actors including Stanley Tucci, Sergio Castellitto and, especially, Ralph Fiennes, who’s front and center as the cardinal charged with overseeing the conclave.

The film begins as a pope has passed. Men surrounding his body shed tears. Prayers are given in Italian. The ring is removed from his hand.

“The throne of the Holy See is vacant,” says a cardinal, John Lithgow’s Tremblay.

Three weeks later, it is the eve of the conclave, and the cardinals are arriving. Before they are locked inside the Sistine Chapel to conduct their all-important business, some share conversations and smoke cigarettes.

A contingent of progressive cardinals wants Tucci’s Bellini, an American, to be the next pope.

“What if I don’t want it?” he asks, adding that no one sane would seek the promotion.

Even if Bellini truly doesn’t want it — about which you have to wonder — he abhors the thought of all of the power that comes with it to go to a highly conservative Italian cardinal, Tedesco (prolific Italian actor Castellitto), who makes impassioned speeches about the importance of tradition in the church.

Sergio Castellitto portrays conservative Italian Cardinal Tedesco, whom progressive cardinals fear will take the Catholic church backward, in “Conclave.” (Courtesy of Focus Features)

Other contenders include Nigeria’s Adeyemi (Lucian Msamati, “The International”), who would be the first African pope, and the ambitious Canadian Tremblay.

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Ralph Fiennes dives into papal secrets in ‘Conclave’

Fiennes’ Lawrence, dean of the Catholic College of Cardinals, will have plenty on his plate, starting with the arrival of an unknown cardinal, the Mexican Benitez (Carlos Diehz, making his feature debut), who has been leading a small ministry in Afghanistan.

All of this responsibility comes as Lawrence has been struggling with his faith, the man having been denied a request by the late pope to leave Rome for a monastery. This relatable questioning of his beliefs seems to be a significant reason why he starts to garner votes himself, a development that, while predictable to the viewer, comes as a shock to him.

“Conclave” is part political thriller — we get wheeling and dealing among the cardinals — and part detective story, with Lawrence investigating potential reasons why this or that cardinal should not be allowed to become pope.

One of the few frustrating aspects of the screenplay by Straughan (“Tinker Tailor Soldier Spy”) is the inconsistency of the Lawrence character; for a significant part of the story, he waffles between being a dogged pursuer of the truth and deciding to stay on the sidelines, content with letting the chips fall as they may.

The former ultimately wins out, leading to engrossing scenes that the steady and measured Fiennes (“The King’s Man”) shares with actors such as Msamati, Lithgow and the always-compelling Tucci (“Spotlight”), who is particularly fiery as “Conclave” builds to a climax.

Although this is, understandably, a male-heavy cast, veteran actor Isabella Rossellini (“Succession”) is impactful as Agnes, a nun key to one of Lawrence’s investigations.

Isabella Rossellini portrays Sister Agnes in “Conclave.” (Courtesy of Focus Features)

That “Conclave” is a feast for the eyes is unsurprising given the cinematic beauty of German filmmaker Berger’s previous effort, the acclaimed 2022 adaptation of “All Quiet on the Western Front.” Notably, though, he worked here not with that film’s Academy Award-winning director of photography, James Friend, but instead with Stéphane Fontaine (“Elle”), trading vast exteriors for lavish interiors in the process. With filming in the Vatican prohibited, the production team created some impressive forgery sets — with production designer Suzie Davies deserving of kudos — all captured nicely by Fontaine.

Also, the pacing of “Conclave” is a far cry from the slow march of “Western Front,” Berger moving swiftly — and deftly — among story threads and aided by myriad instances of crackling dialogue.

“Conclave” also, laudably, swims in thought-provoking thematic waters and explores worthwhile topics including the role of women within the church.

All of the strenths of “Conclave” make it easy to forgive that that narrative occasionally tiptoes to the line of being a little silly. Surely, a certain amount of politicking takes place at these rare events, but you never feel like you’re watching something close to the real deal.

Again, though, the actual journey that concludes with that white smoke emanating from the Sistine Chapel’s chimney surely isn’t this gripping, so “Conclave” gets our blessing.

‘Conclave’

Where: Theaters.

When: Oct. 25.

Rated: PG for thematic material and smoking.

Runtime: 2 hours.

Stars (of four): 3.