(Warning: This column contains major spoilers about the ending of “Once Upon a Time…in Hollywood.” Read at your own risk.)

I want to talk about the ending of “Once Upon a Time…in Hollywood,” starting around the moment the Manson family shows up on Cielo Drive, and…well, okay, we’ll get into it in a bit. So if you don’t want to know what happens during the last half hour of Quentin Tarantino’s new movie, stop reading now or forever hold your troll. This column hinges on some major spoilers, but my desire isn’t to tread on anyone’s pleasure of discovery. It’s to look at a sequence that needs to be looked at, because it’s one of the defining movie sequences of the year.

Before I deal with the ending, though, I want to talk about the first two hours of “Once Upon a Time…in Hollywood” and how totally, addictively Quentin captivating they are. People ask me all the time if I ever change my mind about a movie, and my answer is, “Sure, but not too often.” That’s the truth; I don’t waver all that much. But there’s a subtle, ongoing way that I’m always changing (or at least altering) my mind about a movie, and that tends to happen when I go back and see a film I totally responded to for the second or third time. On that second viewing, especially, you can sort of let go of following the basics — the nuts and bolts of what’s going on — and revel, all the more, in the textures and flavors and character nuances, the world of what you’re seeing. On a second viewing, if a movie is good enough, you sit back and take in the big picture and sort of savor it all, even more than you did the first time. (It’s kind of like going back to an amazing restaurant and discovering that the dish that first wowed you tastes even more delectable.) And that, in its way, is a different experience from the first viewing.

I’ve had that sensation at hundreds of movies I’ve returned to, and last week, when I saw “Once Upon a Time…” again, I had that uniquely entrancing, the-second-time’s-the-charm experience. It’s not that I disagreed with anything I’d written back in May, when I saw the movie during its world premiere at the Cannes Film Festival and posted my review three and a half hours later. I stand by every word of that reaction. What’s changed is that what I originally dug about the film has deepened.

There’s a special feeling Quentin Tarantino’s movies can give you. It’s like a contact high. And with “Once Upon a Time…,” I got dosed even more the second time. I fell a little more in love with what I was watching. I luxuriated in Tarantino’s vision, and grew more obsessively connected to certain moments — like, for instance (I still can’t explain this, but that’s part of the beauty of it), Quentin’s use, for about 20 seconds, of Bob Seger’s “Ramblin’ Gamblin’ Man,” a song I’ve known for 45 years and have never liked all that much. Now, suddenly, I feel like it’s my favorite rock ’n’ roll song of all time. It’s as if the movie made me hear it for the first time. That’s the Tarantino alchemy for you. I could name other moments, but what it comes down to is that for its first two hours, “Once Upon a Time…in Hollywood” is a blissed-out ramble, a Hollywood hang-out movie I connect to on every level: as 1969 L.A. time machine, as intimate backlot showbiz drama, as middle-aged buddy movie, as heady and ineffable Quentin ride.

But it’s partly because I love those first two hours so much that I so deplore what comes afterwards. I’m not mincing words here, because in this case there’s no way to mince them. Simply put: I hate the ending of the movie. And I reject it. It doesn’t connect, in any way, to my philosophical or chemical nervous system. I can understand, on some theoretical level, why Quentin did what he did, but the truth is that it makes no sense to me. And I think, frankly, that the ultimate sin of it is that he wrecked the opportunity to elevate his mesmerizing not-quite-true-life Hollywood dream movie into a great movie.

Even here, I won’t discuss the ending in very much detail. But let’s just reveal what a weekend’s worth of audiences now know: that in “Once Upon a Time…in Hollywood,” four members of the Manson family (Susan Atkins, Tex Watson, Patricia Krenwinkel, and Linda Kasabian) show up on Cielo Drive to carry out Charlie’s orders by slaughtering everyone in the former home of Terry Melcher, the record producer who, in Manson’s paranoid mind, seduced and betrayed him. The house has been rented by Roman Polanski (who’s out of town), and his pregnant wife, Sharon Tate (Margot Robbie), is there, along with her friend Jay Sebring (Emile Hirsch) and several others. We all know what happened next.

But in Quentin’s version, the Manson killers — who seem a little stupider than their true-life counterparts — don’t go to Roman Polanski’s house. Instead, they go to the house next door, where Rick Dalton (Leonardo DiCaprio), the fading TV star who’s the hero of “Once Upon a Time…,” is hanging out with his stuntman and driver, Cliff Booth (Brad Pitt). And what unfurls, from that moment, is a splatterific climax of gruesomely over-the-top violence in which the Manson killers get slaughtered by Rick and (mostly) by Cliff. It’s presented as a kind of cosmic payback of history, with the Manson family getting done to them what they did to others, all in the form of a garishly insane Quentin-tops-the-’70s revenge horror film.

Who’d have guessed it? Quentin Tarantino, the man who revolutionized independent cinema, has made the first dramatic feature about the Manson murders that has a happy ending. Good for him, I guess. And good for us. At least, if you believe that movies should be fairy tales.

This isn’t the first time that Tarantino has “altered history,” of course. But when he did it a decade ago in “Inglorious Basterds,” giving his World War II epic a culminating ambush in which Hitler gets killed in a B-movie inferno out of a Robert Aldrich film, he was winking at the historical liberties taken by big-budget Hollywood pulp filmmakers — and besides, there was a baseline accuracy to the scenario he presented. Hitler went to his death. The Americans led the charge to victory. And that, if you don’t want to get too specific about it, is kind of what happened.

But in “Once Upon a Time…in Hollywood,” Tarantino doesn’t just toy with history (which is a filmmaker’s right). He reverses history and destroys it. In the context of an otherwise captivatingly detailed and authentic work of fiction, he waves his magic wand to create a world in which the Manson murders never happened. It’s almost like his grindhouse version of “Yesterday.” (What if the Beatles had never existed? What if Charlie Manson got stopped in his tracks?) Call it his “Helter Skelter” melter.

So let’s play devil’s advocate: What, exactly, is wrong with doing this?

Well, for starters, Tarantino can make any movie he wants, but he can’t remake the world. And the world we live in today is the post-Manson world. Even at the end of the ’60s, when we thought we’d seen everything, Manson and his family committed crimes that snuffed the last blossom of our innocence. The whole fascination of Charlie and his followers — the reason that Manson, along with Hitler, is the 20th century’s most hypnotic celebrity of death — isn’t just that they were a brainwashed cult of hippie psycho killers. It’s that their savage destruction of empathy (let’s murder these people because…we just don’t give a flying fuck about them!) expressed the underlying dark spirit of our age. The very fact that anyone could do this meant that we lived in a world with people who could do this. That perception changed everything.

“Once Upon a Time…in Hollywood” takes the Manson family seriously. Prior to the night of Aug. 8, 1969, their vibe of destruction is presented as a hidden strand in the Hollywood web, and an element of the film’s drama comes from the note of dread their presence injects into the action, especially in the fantastic sequence when Pitt’s Cliff Booth pays a visit to Spahn Ranch. But that’s why, in treating the Manson killers like villains who can be wiped out with a single bloody video-game flourish, Tarantino violates the rest of his movie — and, in fact, creates a climax that seems to belong to a different movie. I realize he did this on purpose, but that doesn’t mean that it tracks. Watching the sequence, you feel as if the film had been channel-zapped.

Yet there’s one highly perverse way that Tarantino remains “true” to Manson. The mayhem visited upon the Manson killers in “Once Upon a Time…” is brutally horrific, especially when Cliff mashes one of the women’s faces, over and over, into a telephone. Cliff, earlier in the evening, had smoked an acid-dipped cigarette, so on some level this is violence as a sick-joke midnight stoner comedy. But, of course, it’s known that the Manson killers were high when they did what they did, so Cliff’s trippy state also serves to connect them. He’s doing to them what they did to others. And so is Tarantino. He gets to push “rewind” on the Manson family madness yet revel, as a filmmaker, in the violence they unleashed. He’s having his maniac cake and eating it too.

I realize that Tarantino may have painted himself into a corner when he took on the Manson murders in the first place. Did anyone really want to see his authentic staging of them? And I suspect that the view I’m expressing may turn out to be a minority view. I’m not sensing any major outcry against the ending of “Once Upon a Time…” If anything, I’m hearing that a lot of audiences love it, that for them it’s the film finally getting down to the sort of brute action they can relate to, the sort of slasher cartoon catharsis that gets people whooping with glee. But, if so, what are they whooping at? The destruction of those inglourious hippie basterds? Or the destruction of reality? Why does Quentin want to bash the counterculture anyway? It’s not as if the pop and rock ‘n’ roll he reveres was made by studly TV cowboys.

There’s a part of Tarantino that’s always been devoted to a version of reality. There’s another part that’s devoted to lurid schlock taken to the nth power. In a great Tarantino movie, like “Pulp Fiction” or “Inglourious Basterds,” the two sides are in perfect balance. “Once Upon a Time…in Hollywood” is actually more realistic than just about any movie Tarantino has made, yet it’s as if he repressed his pulp-gonzo side until the end, when it erupted like a geyser.

The result, I would argue, smudges the film’s achievement, yet in some insidious way the end of “Once Upon a Time…” is ideally suited to the fake-news era. Quentin makes the trashing of history look hip. And though the notion that Sharon Tate “lives” is supposed to send us out on a feel-good cloud (when, in fact, it’s arguably a trivialization of her memory), the upshot of the film’s defeat of Charles Manson is that Rick, the fading TV star, gets invited up to Sharon’s pad to hobnob with the Polanski circle. “Once Upon a Time…” immerses us in the mystery and the burbling pop excitement of 1969, but by the end it is every inch a movie of 2019, where even a fantasy as world-altering as the decimation of the Manson family is treated as nothing so much as a rockin’ career move.