Joaquin Phoenix as Arthur Fleck and Lady Gaga as Harley Quinn in “Joker: Folie à Deux.” Niko Tavernise/DC Comics/Warner Bros. Pictures

A choral teacher tells a class full of inmates at Arkham Asylum that music can help make them whole again. Cut to Lady Gaga’s Harley Quinn gazing over her shoulder. Is Arthur Fleck too broken, or will she get a taste of deranged bliss with the clown-faced killer?

The next minute she’s coarsely cooing “Get Happy,” the ecstatic ode made famous by Judy Garland, through plumes of cigarette smoke, and it seems we’re about to find out. But “Joker: Folie à Deux,” which waltzed off a cliff at the box office on its opening weekend, is as hopelessly fractured as poor Arthur is. The erratic musical numbers can’t save it, but not because they don’t belong.

The splintering is part of the point: The $200 million sequel zooms in so close on a shattered consciousness that it’s impossible to tell what’s happening and what’s a distorted reflection. Joaquin Phoenix, who won an Oscar for 2019’s “Joker,” has said the idea for the follow-up came to him in a dream. Director Todd Phillips suggested they toyed at first with making a Broadway show. (You think fanboys are mad now?)

Musicals are wide-open playgrounds for frolicking through the psyche’s deepest recesses: Reckoning with painful pasts, exploring dual identities and falling deliriously in love are all classic stuff. Many do all three and much wackier besides, including “Wicked,” “Phantom of the Opera” and “Into the Woods,” all of which have been adapted for the big screen. Musicals were made to tell a story like Arthur’s, so what went wrong?

Arthur’s thirst for the spotlight predates even his bloodlust. Remember Phoenix gyrating his spindly arms in full maroon-suit regalia on those Bronx steps to the tune of “Rock and Roll Part 2” in the first movie? (Eddie Redmayne’s Emcee in “Cabaret” could never.) So why not give the man his big break, even if it is behind bars and probably all in his head?

I’m not a stickler for rules when it comes to why characters sing or dance. As long as they’re into it, their voices aren’t bad (or at least not grating) and they approach the vicinity of making me feel anything at all, I say sing out, Louise. But most of the songs in “Joker: Folie à Deux” fail all three of these low hurdles, and the ones that don’t are too little, too late.

Before Little Monsters claw into my DMs, Gaga sounds glorious on “Gonna Build a Mountain,” a reference to the sweet if nonsensical dream that she and Joker share should they run off together. Like the movie’s other more successful numbers, it’s a set piece that shows off where the budget went. And it’s one of more than a dozen familiar songs in the film, including “I’ve Got the World on a String,” “Bewitched” and “That’s Entertainment,” drawn from the Broadway and Hollywood vaults.

Gaga covers these standards and more on her companion album, “Harlequin,” using the full might of her talents. But on screen, she affects a rough sort of talk-singing, on par with what Phoenix musters, that inspires little beyond a regretful cringe. We know how much better she could sound. These are both wounded, vulnerable people, but the tentativeness in their voices conveys nothing so much as a hesitancy to embrace the form, like no one is sure they want to be making a musical. When the tunes are in your imagination, why not sing your face off?

Unappealing though they are, the half-baked vocals seem meant to communicate emotions that are otherwise underexplored. The connection between Joker and Harley doesn’t appear rooted here in chemistry, eroticism or any sort of meeting of like minds. (It’s very possible she exists only in his, for how easily she keeps bypassing prison guards to make out.) The movie rather seems to assume we know their fates are tied because of how many times their stories have been told before.

Showmanship has long been part of Joker’s on-screen persona. Jack Nicholson twirls a cane to the rhythm of “Partyman” on a museum raid in 1989′s “Batman,” for which Prince performed the hit soundtrack. When Harley Quinn was introduced in 1992, in one of my after-school favorites, “Batman: The Animated Series,” she and Joker were no strangers to musical escapades around Gotham. An affinity as twisted and gleeful as theirs seemed ideally suited to gallivanting in song. A musical might have been just as apt to probe their darkest inner shadows. But “Folie à Deux” fumbles the only real principle of show business: commit to the bit.