Arthur Fleck works as a clown and is an aspiring stand-up comic. He has mental health issues, part of which involves uncontrollable laughter. Times are tough and, due to his issues and occupation, Arthur has an even worse time than most. Over time these issues bear down on him, shaping his actions, making him ultimately take on the persona he is more known as...Joker.

After being kicked out of his rock band, guitarist slacker Dewey Finn faces a mountain of debts and depression. He shares an apartment with his best friend, Ned Schneebly, an old band member who is now a substitute teacher. Dewey accepts a job as a substitute teacher at a snobbish private elementary school where his attitude and hijinks have a powerful result on his students. He learns they are talented young musicians, and he decides to form a rock band with them to win the $10,000 prize money...