The final moments of Todd Phillips’s “Joker” (2019) remain among my most terrifying moments at a movie, as much for what was happening in the theatre as on-screen. As Joaquin Phoenix’s Arthur Fleck gave into the madness of his costumed alter ego, Joker, and shot out a preening talk-show host’s brains on live TV, the crowd at my screening erupted in applause and a man a few seats down from me leaped to his feet, screaming “Yes!” and pumping his fist in triumph. There was a lot of loose talk around the release of “Joker” about the hero-ification of psychopathy and the specter of copycat violence, but in that instant, I saw a malignant spark arc from screen to audience, and I was afraid.

Maybe Phillips was, too, because “Joker: Folie à Deux” seems like an act of atonement for the perceived sins of the first film. It’s a movie of ideas disguised as an exceedingly grim entertainment, and it’s more interesting FOR those ideas, undeveloped though they are, than it is as entertainment. It essentially says to that guy at my “Joker” screening: “Sit back down. Now.” Which may strike one as disingenuous, but better late than never.

Oh, and this new “Joker” is also a musical – take THAT, fella – and one that audaciously uses the Great American Songbook as a template of resentment and revenge, teasing out the dark undercurrents of such classics as “Get Happy,” “Bewitched, Bothered and Bewildered,” and (ahem) “That’s Entertainment.” That the movie embeds its musical numbers – unfolding mostly in Arthur’s damaged mind but given the full MGM top-hat-and-tails treatment – in a dank, overlong jailhouse/courtroom melodrama is a rich notion that goes nowhere other than to underscore Arthur/Joker’s split personality with a very thick Sharpie. But if it gets the kids listening to Ella or Sinatra, who are we to kick?

As the title implies, “Joker: Folie à Deux” gives Arthur a playmate in the form of Lee Quinzel, a.k.a. Harley Quinn (Lady Gaga), a fellow inmate at the Arkham State Hospital for the Criminally Insane and Joker’s biggest fan. It’s she who awakens the hero’s inner Fred Astaire, and the couple’s musical duets are a fascinatingly lopsided highlight of this very strange film. Gaga can sing, obviously, even when she’s trying to sing poorly, and Phoenix can barely hold a note even when he’s trying to sing well; those crosscurrents lend a note of real pathos to their musical numbers, like a jailbreak sequence set to a literally incandescent rendition of “If My Friends Could See Me Now.”

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Joaquin Phoenix and Lady Gaga in a scene from “Joker: Folie à Deux.”Photo by Niko Tavernise /THE ASSOCIATED PRESS

The asylum scenes, with Brendan Gleeson in clover as a gloating tormentor of a guard, have a grimy, propulsive energy that grinds to a halt in Arthur’s trial for the murders of the first film. (They also focus weirdly on the characters’ cigarette smoking, to the point that you wonder if Marlboro has a piece of the movie’s action.) Taking up the majority of “Folie à Deux,” the trial is shot in doomy shades of brown, with Catherine Keener as Arthur’s beleaguered defense attorney and legions of Joker fans in near riot outside. At issue is whether Arthur was in his “right mind” when he killed those people – was he the cringing nobody Arthur Fleck, or Joker, filled with a raging, righteous fury at a society bent on crushing the nobodies?

Phoenix remains the primary reason to give this franchise your attention. The “Joker” films are the movie industry’s most extreme case for comic-book cinema as serious, Oscar-worthy art, but the very attempt courts pretentiousness, and the star is the only one to escape it. Phoenix’s Arthur remains an indelible creation, gaunt shoulder blades jutting out like the broken wings of an angel and a laugh that could give Kafka nightmares. That laugh – a convulsive rictus that’s both a howl of defiance and a whimper of defeat in the face of the universe’s cruelty – is the most emotionally devastating aspect of both “Joker” movies. It’s the cry of a man beaten not just physically but existentially, and it haunts a viewer well past the end credits.

By contrast, Arthur’s homicidal alter ego is the reason the fans roil in the streets. Joker is why the psychotic Harley Quinn has gone gaga (she has no interest in Arthur), and he’s why that guy I mentioned up top went nuts during the first movie. Joker has harnessed the power of Arthur’s underdog anger, and he promises the liberation of anarchy, on-screen and off. (If you want to make any connections to current political figures, be my guest; the movie doesn’t.) “Folie à Deux” tries to walk all that back and rub our faces in the hypocrisy of a culture that lionizes charismatic killers – which itself is an act of hypocrisy that, honestly, boggles the mind.

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Joaquin Phoenix and Lady Gaga in a scene from “Joker: Folie à Deux.”Photo by Niko Tavernise /Warner Bros.

Still, rare is the blockbuster movie that consciously self-destructs and tries to take its audience along. It will be interesting, to say the least, to see mainstream moviegoers’ reaction to a film that punishes them for their daydreams of revenge; I doubt it will be pretty.

If superheroes function as a power fantasy for us impotent humans, supervillains enact something darker and more seductive – a fantasy of using that power to even the score with a world that refuses to see us as we see ourselves. With more daring than success, “Joker: Folie à Deux” says that anyone who takes the Joker for a hero to be emulated is as delusional as Arthur Fleck, and it serves up its comic-book cake at the same time it stuffs it with rat poison.

All together now: That’s entertainment!

RATING: **1/2 OUT OF FOUR