Mid-eighties of the last century, Gotham. Arthur Flack, who dreams of becoming a comedian, suffers from a mental illness, as a result of which he has uncontrollable fits of laughter. But the man does not have a good sense of humor, and his numbers do not resonate with the audience. He is single and takes care of his elderly mother. Attempts to succeed lead nowhere, but Arthur does not intend to give up. The host of a popular TV show ridicules Flack's attempts to create a good stand-up number, and the hero decides to commit suicide on the air. However, the humorist's caustic statement finally breaks something in the psyche of the loser, and he begins his bloody march to fame and madness.