Folie à deux — a shared madness — is a natural theme for a musical romance between Batman characters, but in Todd Phillips’ rollicking follow-up to his 2019 megahit “Joker,” the first question you’ll have is: Who are the two herein foil? Is it Joker and Harley Quinn? Arthur Fleck? Anyone else? While momentum issues and a general heaviness haunt the film, the film does offer some interesting possible answers, but none to the general question: what did I just see and why?

“Joker: Folie à Deux,” co-written by Phillips and Scott Silver, picks up two years after “Joker,” with Arthur Fleck (Joaquin Phoenix reprising his Oscar-winning role) in Arkham Asylum, awaiting trial for his many crimes. The action is preceded by a Looney Tunes-esque cartoon entitled ‘Me and My Shadow’, which depicts Arthur being stalked by his own shadow, which is more powerful than he is. It foreshadows both the musical slant of the film and a metaphor for the unconscious and often unacceptable parts of ourselves. Arthur’s dedicated lawyer (Catherine Keener in yet another intelligent, empathetic turn) prepares a “shadow” defense: Arthur is innocent by reason of insanity, because Joker is a separate personality within him.

That’s one of the film’s possible duos. But soon, Arthur will meet fellow inmate Harleen “Lee” Quinzel (Lady Gaga), the movie version of Harley Quinn, previously played by Margot Robbie in “Suicide Squad,” “The Suicide Squad” and “Birds of Prey.” These crazy kids love music and talk about killing their parents. The rest of the plot concerns Arthur’s trial, prosecuted by Gotham’s slick Assistant District Attorney Harvey Dent (Harry Lawtey). Oh, and there are songs. Lots of songs.

Lady Gaga and Joaquin Phoenix in the film “Joker: Folie á Deux.”

(Niko Tavernise / Warner Bros. Images)

“Folie” is every bit a sequel to “Joker,” told in the same heightened cinematic language, with key collaborators returning. Cinematographer Lawrence Sher captures arresting images: Arthur’s reaction when Lee drew a lipstick smile on glass; or Arthur and Lee exchanging cigarette smoke through prison bars. Phoenix is ​​just as intensely involved as we are used to and Gaga convinces as a crazy super fan. But of course the biggest change is that they regularly burst into song (sometimes it’s less bursting than leaking) and scouring the Great American Songbook for songs like “That’s Entertainment!” and “Bewitched.”

There are 11 songs on the soundtrack album and more than that in the film, presented in whole or in pieces. The numbers sometimes serve a narrative purpose, other times not so much. The performers are wild, but these musical breaks test our patience. They often break the momentum, and there isn’t much of that early on, with no attempt to create tension about the verdict or any grand plan that Arthur or Lee might come up with. That lack of cheerful masterminding is a painful reminder that this isn’t anything close to “Joker.”

The 2019 film was more like a bastard child from ‘Taxi Driver’ and ‘The King of Comedy’ (Robert De Niro was even in it) than any traditional origin story; Phillips was clearly more interested in emerging from a Martin Scorsese template than a DC version. (In the sequel, a judge character looks suspiciously like Scorsese.)

So what, aside from another billion dollar gross, motivates the second film? The audience will wonder about this for almost the entire film ‘Folie à Deux’, which usually starts in fits and starts until a tour de force in which Arthur/Joker serves as his own lawyer. That’s Phoenix at its best and the movie at its best. It’s hilarious, strange and menacing in just the right way – and a hit for a somewhat lagging comic book movie. But those waiting to see Arthur become history’s insane Joker will do so in vain. This isn’t about that either.

It’s also not about the romance, which is not explored in depth. The film seems more concerned with what others – the media, the crowd outside the courthouse – expect from Joker. It’s so disconnected from the supervillain story that it’s almost meta. But since the musical numbers become long detours rather than providing Arthur with further insight, the sequel doesn’t sing as a character study. And it’s certainly not a thriller.