Wow, I really disliked this movie.
It's the tale of poor, mentally ill, Arthur Fleck, doomed to be a loser in the hellhole that is Gotham City. All he wants to do is to be a standup comedian. But unfortunately he's not funny. And he doesn't understand enough about people to even act funny. And part of his mental illness is his propensity to laugh uncontrollably at inappropriate times.
So he becomes a homicidal maniac. Hey, who wouldn't?
Now, there's a Batman tie-in, of course. Bruce Wayne's father, Thomas, is portrayed as an uncaring plutocrat. (Think Mike Bloomberg, except more explicit in his disdain for the less fortunate.) The only thing to wonder about is whether Fleck is gonna shoot him in front of Bruce, or someone else. (Spoiler, because I don't care: someone else.)
The details are thoroughly unpleasant, and go on far too long. But your mileage may vary, as it did with the Oscar folks; they nominated it in a bunch of categories, including Best Picture. And Joaquin Phoenix won best actor.
And the IMDB raters have it (as I type) as number 48 on the best movies of all time. Sheesh. Maybe I was just in a bad mood.