“Sidney Chambers is a passionate man,” Alan Cumming said as he introduced the fourth season of the British detective series “Grantchester,” which concluded last Sunday, on PBS’s “Masterpiece: Mystery!” For the first three seasons and the top of the fourth, James Norton starred as Sidney, a kindhearted, single, and babelicious Anglican priest in cozy nineteen-fifties Grantchester, in Cambridgeshire, where locals love nothing more than a festive church fête, and where crimes are solved by a cop and a vicar. Yes—like many British vicars and priests, Sidney investigates murders. Thus, as Cumming explained, his Scottish accent on full blast, Sidney is a minister who knows grace—and a detective who knows evil. “Sometimes the man of the cloth struggles with the man of the world,” Cumming continued. “Especially when love”—head toss, eyebrows up—“and murder”—head down, disturbed squint—“are involved.” If you had watched the first three seasons of “Grantchester,” Cumming’s introduction could make you feel a bit exposed: you knew this show flirted with the ridiculous; Cumming knew it; the introduction knew it. Worse yet, you were on tenterhooks, anxious for more. This season, Norton would leave the show and a new easy-on-the-eyes vicar would come to town. Could “Grantchester” bear it? Could we?

“Grantchester,” which premièred in 2014, is based on a series of short-story collections by James Runcie, whose father, Robert Runcie, was the archbishop of Canterbury. The books were inspired by Robert Runcie’s milieu, if not his adventures, after the Second World War. James Runcie has written that he wanted his stories to “trace how modern Britain evolved” in that era, balancing the good—social progress—with the bad, including “the decline of community, selfishness, intolerance, racism, homophobia, crime.” For TV, he wanted a “sexy vicar” to enhance the appeal of a theoretically unfashionable lead: a practicing Christian. In its highly enjoyable first three seasons, “Grantchester” knocked at least one of Runcie’s objectives out of the park. James Norton looks fantastic in cassocks, suits, and a priest’s collar; his sensitive hunkiness greatly enhances the appeal of this particular reverend, who, for good measure, is also a Scots Guards veteran who broods, fornicates, drinks whiskey, wheels around on a bicycle, and listens to unbelievable amounts of jazz, a genre that only he, in all of sleepy Grantchester, understands. (Sidney’s peers chuckle about his love of Sidney Bechet, often mispronouncing it as “Beckett,” to indicate that villagers who aren’t Sidney ne comprennent pas.)

Though he lives among two Christian squares—his flinty housekeeper, Mrs. C. (Tessa Peake-Jones), and a shy, closeted curate, Leonard Finch (Al Weaver)—Sidney’s boozy, war-haunted soul is better suited to the hardboiled company of Geordie Keating (Robson Green), a middle-aged police detective. Whenever a swell is stabbed to death in a garden or a university lecturer falls off a chapel spire, Sidney is by Geordie’s side, handsomely Sherlock Holmes-ing it. On those journeys of bludgeoning and self-discovery, he ministers to the devastated, his progressivism gently protecting people from bigots and wearing down Geordie’s fustiness. He’s sexually astute, as well—comfortably modern, neither a creep nor a prude, unfazed by same-sex attraction or by parishioners’ scandalous confidences. (Like his Bechet, Sidney’s empathy is meant to stand in for our own comparative enlightenment.) He’s hopelessly in love with Amanda (Morven Christie), who marries someone else, tumultuously; when tormented, he consoles himself by respectfully ravishing comely widows and night-club singers. Sometimes he tends to the cemetery’s landscaping, sweaty and scything something; once, he begins the season by entering and emerging from a swimming hole. A lesser actor might have felt objectified, but Norton bore it admirably, abs gleaming in the sun.

Whether “Grantchester” glamorizes Christianity, though, is a matter of ongoing philosophical inquiry. For one thing, we never really understand why Sidney chose to become a priest: in one episode, we see him gazing at the Bible he had when he was ten, but he seems to be in the clergy for the compassion-fostering more than for the Lord, Himself. Episodes end with rousing plot-themed sermons that most seculars would happily endure: the compelling goodness that Sidney radiates is connected to ideals and to faith but less specifically to Jesus. Narratively, this state of affairs was ideal—no offense, Jesus—but unsustainable. The unlikeliest conceit on “Grantchester” turned out not to be the obvious one—a cop-vicar sleuthing team in a quaint village beset with murderers—but that the vicar was a vicar at all. Throughout, and especially by the end of Season 3, Sidney Chambers, as written and acted, would have had little reason to believe that Anglican leadership was the best vehicle for his compassion.

For most of its existence, “Grantchester” deftly balanced episodic, procedural goings on with deeper character arcs: Sidney’s struggle to find love, mostly against the wishes of the church (a no-divorcées-for-vicars rule); Leonard’s struggle for love and self-acceptance, against the wishes of church, law, and society; and the atheist Geordie’s struggle to be a good husband and father. By late in Season 3, all three of those arcs come to satisfying fruition—but, for two of them, the church was a problem. In the Season 3 finale, we fervently wished that Sidney would defrock himself, for love, and he almost did—but the frock was the one garment the show refused to remove. After three seasons of promoting love and sticking it to intolerance, the show abruptly and unconvincingly reversed course and tried to act the nobler for it. After that, Norton started filming a show called “McMafia” and left “Grantchester.” Take that, Church of England.

“Grantchester” is popular; it airs in more than a hundred and thirty countries, and is streaming on Amazon. The show pluckily forged on without Norton (New vicar? No problem!) and took its existential crisis to new extremes. Season 4 begins by giving Sidney a hasty, unconvincing new romance—and, worse, does so via a self-serving use of the American civil-rights movement. He moons over Violet (Simona Brown), the daughter of a visiting civil-rights leader and preacher from the American South, seduces her on the night her brother is murdered (“She’s grieving, for chrissake!,” Geordie says, speaking for all of us), and ships off to Dixie, with all the grace of Poochie getting sent back to his home planet. All of this is terrible. (Violet doesn’t even seem to like him.) So is his unconvincing goodbye with Geordie. (Come visit—bring the kids!, Sidney says. Sure, guys.) Watching it, I pitied the actors and felt implicated in the show’s ridiculousness even more than Alan Cumming’s intro had suggested I would. Like Season 8 of “Game of Thrones,” watching parts of Season 4 of “Grantchester” made you feel yanked around, your intelligence underestimated. In subsequent episodes, as we bob along confusedly in the wake of Sidney’s exit, the new handsome vicar, Will Davenport (Tom Brittney), zooms up on a motorcycle, in leather, ready to unsheath his magnifying glass.

Why is any of this happening? one wonders. The reeling continues throughout the season, even as more locals are murdered and clergy consulted, in the manner we once loved. The dawn of rock and roll helps a bit: a Richard Lester-like chase sequence set to “Long Tall Sally,” the amusing spectacle of Geordie railing against Teddy boys. (Next season—it’s been renewed—I’m hoping for skiffle.) But the remaining regulars fumble along, behaving a bit more improbably than they used to, and meanwhile, we have to learn about Will, whose characterization, despite Tom Brittney’s likable, thoughtful presence, doesn’t quite hook us with intrigue. Will’s biographical details are doled out dutifully, with a hint of mystery—ooh, why does he know sign language?—but not enough. He’s a recreational boxer. (Sometimes he punches people, which vicars shouldn’t do.) He’s the heir to a giant country house. He’s celibate, which is disappointing: the only relationship he’s looking for, he says, is with the man upstairs. (And not in a fun way.) All of this has left me agnostic; where I was once a fervent observer, “Grantchester” has made me a skeptic. But, during my spell of freethinking, I’ve come up with some Apocrypha. Sidney didn’t move to Alabama with a woman we knew for two episodes; he left the church with conviction, at peace with himself, and moved to another town with Amanda, whom he loved for three seasons. There, in that charming but more cosmopolitan town, he reads and writes mystery novels, owns a jazz club, and counsels lost souls over whiskey—no frock required.