While waiting for his trial for the murders committed in Joker, Arthur Fleck (Joaquin Phoenix) is being held in Arkham Asylum, where he meets Harleen 'Lee' Quinzel (Lady Gaga) in a music therapy group.
It's love at first sight and, when she gets out before him, Harley promises to be at Arthur's trial every day, as well as vocal campaigner for his freedom.
Arthur's lawyer, Maryanne Stewart (Catherine Keener) seeks to build a defence on the idea that Arthur and Joker are two separate personalities, brought on by his abusive upbringing.
Meanwhile, the district attorney, Harvey Dent (Harry Lawtey) is seeking to dismantle this idea, and is calling for the death penalty for Arthur.
Eventually, Arthur - egged on by Harley - grows tired of his lawyer's approach and dramatically fires her in the middle of the case, opting to defend himself: in full Joker make-up.
I can totally get why people were disappointed with Joker: Folie à Deux. It wasn't the Joker film they were expecting. Not that writer/director Todd Phillips had made any promises about where the sequel would be going... it just wasn't the stylish and violent remix of The Dark Knight that I think people were hoping for.
Rather it turned out to be an arthouse courtroom drama - and character piece - infused with elements of One Flew Over The Cuckoo's Nest (the Arkham scenes) and Natural Born Killers (the delusions and the media's glorification of murderers).
In truth, Phoenix's Joker is nothing like the source material (80-plus years of comic book escapades), but more akin to someone cosplaying as The Joker.
As I said in my review of 2019's Joker (earlier today), the character has always worked in the comics, and most other media, because he doesn't have a definitive origin story. He's an enigma and that makes him more frightening - he's the one mystery the world's greatest detective, Batman, can't solve.
But Todd Phillips and Joaquin Phoenix have made the character all too human, with a name, family and origin story, and - especially in this sequel when he is often depicted as - frail and wracked with self-doubt.
That makes for an interesting character... he's just not The Joker.
He is a straw man held up as a figurehead for a bubbling, anarchic revolution in rundown Gotham City, but - unlike the comic book iteration or even any of the live-action takes, right back to Cesar Romero in Batman '66 - he comes to the conclusion that he doesn't want that mantle.
Which, again, makes for an interesting revelation, but it just doesn't feel like something The Joker would do.
Providing strong support is Lady Gaga as a low-key, grounded Lee Quinzel, who gets under Arthur's skin and fuels his frequent delusions and dreams that frame themselves as musical numbers (one in the courtroom is very reminiscent of Sid Vicious' performance of My Way at the climax of The Great Rock 'n' Roll Swindle).
Both Lee and Fleck are complex characters, well developed and growing towards a surprising - yet wholly believable - twist in the final scenes, that pretty much guarantees there won't be a third film in this particular franchise.
Joker: Folie à Deux (a psychological term for a shared delusion) is as gritty as the first movie, presenting us with a beautifully decaying Gotham City, with a troubled element of the populace inspired by their own perceptions of the murderous Fleck.
However there's no getting away from the overwhelming feeling that this was originally a psychological study that has just been dressed up with a few affectations from the Bat'verse to sell tickets.
Which I gather it didn't.
It was a brave experiment. But, sadly, it failed.
I actually believe if this duology has been made with different - or even original - protagonists audiences (coming to the story without any expectations) would have liked it more, because they would have then felt very clever pointing out: "oh, that's a bit like The Joker and Harley Quinn from the Batman comics".
While Folie à Deux lacks the kick of the first film, I did actually enjoy it for what it was, but it was still a peculiar take on the characters that bore little or no resemblance to the general public's vision of the source material.