From Patrick Holman at The Moderate Voice:
Few films this year will unsettle, disturb, and make audiences think as much as Joker.
Starring Joaquin Phoenix, Joker focuses on the less-fortunate life of Arthur Fleck, who is newly unemployed, lives with his mother, and deals with an array of mental illnesses. One of them is an uncontrollable, maniacal laugh that Fleck can’t help but emit when he’s feeling other, unrelated emotions. To say the least, most people are uncomfortable around him, and Arthur Fleck is seen as an outcast in society.
Following his firing, however, Fleck goes off the deep end and slowly descends into insanity and violence. He is Joker, as we already know. The rest of the film gets more violent, more insane, and more unsettling.