The movie “28 Years Later: The Bone Temple” is supposed to be a sequel to “28 Years Later,” the third installment in the film industry’s hottest new zombie film franchise, and strictly speaking, it is. The movie opens right where the last one left off: Our intrepid young hero, Spike, who represents hope for a zombie-free world of the future, has been trapped inside Jimmy Crystal’s murderous psychopathic cult. He’s poised to be executed but manages to triumph. The scene is grisly. His executioner, a boy not much older than he, in fact, suffers a lower-leg wound that becomes an arterial geyser, and he bleeds to death. Under the rules of Jimmy’s gang, Spike’s life is spared, but he’s still the cult’s captive and must serve its leader or risk death again.

One thing is clear right away in this fourth installment of a movie franchise that began in 2002 with “28 Days Later.” For all its focus on zombies, there aren’t very many of them to be found in “The Bone Temple.” Most of the malevolence comes from human survivors of the zombie apocalypse, which has turned society upside down and left everyone — especially the young — morally adrift, without purpose, bound together by no other rules than sheer survival and bloodlust, yet in search of group bonds. Jimmy Crystal, played by Jack O’Connell, is himself a child victim, whose priest-father was slain by the zombies in one of the earlier films and who has spent 20 years morphing into a full-blown anti-Christ. Only Jimmy doesn’t think so. He fancies himself a self-styled savior — dubbing himself “Lord Jimmy” — and his flock does his bidding, or “charity” as he calls it, which is nothing less than pure mayhem.

Jimmy’s also a real hoot. He wears an upside-down crucifix as a symbol of his spiritual authority, and a blond wig, as do all of his young charges. This is a bit of an inside joke, a riff on Jimmy Savile, the popular host of daytime children’s shows in Britain who was later revealed to be a serial pedophile protected by the country’s ruling elites. Savile wore blond wigs and conned the entire country for more than 40 years. He was a charming monster, and so is Jimmy Crystal. Their common bond is their predilection for children. Zombies, victims of the rage virus, have no choice but to prey on humans instinctively, but “Lord Jimmy” does so too, and he diabolically pursues and covets them — the young, the most vulnerable — in the name of human “salvation.”

Jimmy Crystal’s symbolic destiny, evident from his initials and his Christ-like last name, will become clearer by the film’s end, when he’s actually crucified in the Bone Temple and cries out to God for forgiveness. The Bone Temple is laden with religious symbolism, too, as Christ himself was crucified in a place called Golgotha, which translates as “place of the skulls” in Aramaic. In the movie, the Bone Temple is the sanctuary of a solitary figure, Dr. Kelson, played by Ralph Fiennes, who sees himself as the guardian of the souls and memories of the pre-zombie world. But Kelson is also a doctor and has been seeking a cure to the rage-virus that turns humans into zombies and compulsive murderers. Fiennes first appeared in “28 Years Later” but here emerges front and center as one of the film’s leading men. If Jimmy is a charming and charismatic devil masquerading as Christ, Kelson is the opposite: an ugly old sage who dresses in rags, is covered with grime and blood, and looks positively ghastly. He’s not Christ — he’s more like John the Baptist, walking the world alone, hoping for a miracle, and appealing to men’s better angels.

There’s a third figure in this mise-en-scène: a giant of a man (dubbed Samson by Kelson) with the rage virus, whom the doctor seeks to “treat.” When they first encounter each other, Samson is on the attack, but Kelson hits him with a blow-dart filled with morphine. Stopped dead in his tracks, Samson proceeds to mellow and the two become friends (well, sort of). These are some of the most poignant— and humorous — scenes in the Bone Temple, as we witness the good doctor and his semi-recovered “patient” laughing, dancing, and generally cavorting with Nature. Samson, it turns out, also has a huge penis, which the movie leaves uncovered, as well as muscular rough-hewn limbs and a body covered head-to-toe with hair. The movie revels in Samson’s robust physicality; he’s no scrawny or diseased zombie but a symbol of outsized strength; his nakedness conjures up Adam in the Garden, a primordial man who has no shame, and, therefore, no need to clothe himself.

Samson’s freely bouncing penis is also a bit of a sight gag, much like the ridiculous blond wigs worn by the “Jimmies.” For all the terror and blood and gore presented here — the killings, though few, are truly grisly — “Bone Temple” can’t help but inject some levity, as if to remind viewers that the movie has elements of zany fun and it’s okay to laugh. (We do, but not for long, before another human spine is grotesquely ripped from its torso).

It all comes to a frenzied head in the movie’s chaotic Shakespearean final act, as Jimmy and Kelson square off for the third time, while Spike and Samson come to the rescue. Still, “Bone Temple” doesn’t really have a decisive ending, or rather, it leaves open the possibility of another sequel, by refusing to tie up narrative loose ends. Jimmy, in theory, ascends to Heaven or descends to Hell at the end, but one suspects that a resurrection — “28 Years and Three Days Later”? — is still in the offing. Jack O’Connell is so compelling in this role that killing him off could kill off the franchise for good.

Thematically, the great unanswered question is whether the zombie rage-virus can be “cured” and the mayhem finally put to rest. Will Samson’s transformation prove lasting, and will other infected zombies respond similarly? Kelson was running out of morphine, so his promising experiment was in peril. So addressing this question could form the basis of another film, or perhaps several. Why stop now when the franchise seems to have regained its footing and is back on a roll?

Kudos go to the film’s director, Nia DaCosta, a 36-year-old Black woman and rising star herself, who seized the reins from franchise founder and “28 Years Later” director Danny Boyle, and managed to craft a compelling narrative and elicited memorable performances from its ensemble cast. Whether the franchise moves forward at this point depends on how well “Bone Temple” fares at the box office. Receipts from its predecessor were considered disappointing, but fans of the franchise are giving rave reviews to the “The Bone Temple,” earning it a 92% rating on Rotten Tomatoes.