“Joker: Folie à Deux” is a Great American Songbook jukebox musical grafted onto the Grand Guignol of Todd Phillips and Joaquin Phoenix‘s vision of the comic book character.

It doesn’t work, doesn’t play and doesn’t coalesce into the Big Message movie one and all hoped they’d get out of it.

But with Phoenix dueting with Lady Gaga, who joins him as Harley Quinn, and even throwing in a fair approximation of tap dancing into the mix, the film manages to be a fascinating failure.

There’s ambition along with giggles-inducing pretension and grimacing violence. And Phoenix commits to the part and the picture down to his last corpuscle.

Aspiring comic turned mass murderer Arthur Fleck, aka “The Joker,” is in Arkham Hospital for the Criminally Insane, where he’s tolerated, taunted and abused by the guards, especially his handler, Jackie Sullivan (Brendan Gleeson).

“You got a joke for me today?”

If Arthur does, that gets him a cigarette.

Two years have passed since unfunny Arthur’s spree killing, which climaxed with the clown-faced comic murdering a talk show host live on national TV.

One fateful decision made for this film was in not giving us any flashbacks to refresh our memories about what put Arthur in prison. Not one scene from 2019’s “Joker” is served up. DeNiro played smarmy, bullying TV talker Murray Franklin, in case you forgot.

Fleck has been the subject of a TV movie and mass cosplaying fandom. People can’t get enough of this bad comic/unhinged killer. But is he sane enough to stand trial? His attorney (Catherine Keener) tries to get him on board with “just be yourself” and medical science will come to the right conclusion.

A TV interview (Steve Coogan plays the unsympathetic interrogator) might help his cause.

But Arthur has stumbled into his soulmate. Lee Quinzel is in the “singing” wing of the 1930s vintage hospital, where patients sing everything from bluegrass spirituals (“Will the Circle Be Unbroken?”) to standards made famous by Judy Garland (“Get Happy”). Lee (Lady Gaga) stands out in the chorus. And Lee is Arthur’s biggest fan.

They share a smoke, then a song and then a romance as she conspires to bust them out. Maybe on “movie night.” Sure, they’d miss the end of the Astaire/Cyd Charisse/Nanette Fabray and Oscar Levant musical “The Band Wagon.” But where there’s smoke, there’s fire.

“Escape” is not what we’re set up for here. This movie is about the trial, the cultish mania of celebrity and the soul-crushing effect it has on the weak-minded, from John Hinkley Jr. to Kardashian idolizers to prison pen pal brides and MAGAs.

The screenplay and Gaga’s performance do the best job yet of connecting Lee’s conversion to the villainess Harley Quinn as an obsession that becomes a breakdown for a woman who becomes Arthur’s mirror image.

Craving fame and connection to it does them both in.

As “monster/martyr” Arthur sits through a stunningly-dull and straightforward trial, he imagines sharing a “Sonny & Cher” styled variety show with Lee — duets, dances, and loving/insulting jokes.

But while the “Batman” universe movies have always been better at finding topicality in their stock characters and pulp storylines, “Folie à Deux” never comes close to achieving the core aim of all these “dark” comic book adaptations — depth.

Musical flourishes — “When the Saints Go Marching In” is sung, referenced by a hospital exercise yard trumpeter, Gaga solos The Carpenters’ “Close to You,” and the rights of everything from “For Once in My Life” and “What the World Needs Now” to “Bewitched, Bothered and Bewildered” to “That’s Entertainment” were purchased and repurposed.

But to what purpose?

Humorless, shallow and grim will get you only so far. And usually, you don’t have to take tap dancing lessons to pull that off.

When the message is this muddled, all that’s left is to beg the question — “That’s Entertainment?”

Rating: R, violence, sex, smoking, profanity.

Cast: Joaquin Phoenix, Lady Gaga, Brendan Gleeson, Zazi Beetz, Steve Coogan and Catherine Keener

Credits: Directed by Todd Phillips, scripted by Scott Silver and Todd Philips, based comic book characters created by Bob Kane, Bill Finger, Jerry Robinson, Pual Dini and Bruce Timm. A Warner Bros. release.

Running time: 2:12