The film features covers of classics like "Get Happy" and "For Once In My Life," exploring the Joker's delusional world through music.

It opens with an animated Looney Tunes sequence, setting a whimsical yet dark tone. Arthur, portrayed by Joaquin Phoenix, navigates a red carpet event, haunted by his shadow. His relationship with fellow Arkham inmate Lee (Lady Gaga) becomes pivotal, blossoming during a choir performance where she inspires him to embrace humor after a long absence.

Faced with the death penalty, Arthur finds hope in "For Once In My Life." As their bond deepens, he stops taking his medication, leading to hallucinatory experiences reminiscent of classic musicals. Lee ignites the asylum, singing about their imagined paradise.

In a tense interview, Arthur professes his love for Lee through "Bewitched, Bothered, Bewildered," exposing his inner turmoil. During his trial, he fully embraces his Joker persona, performing "There’s Always a Joker in the Pack."

After resting his defense, Arthur returns to Arkham, transformed and welcomed by fellow inmates who rebel against authority while singing "When the Saints Go Marching In." However, the fantasy shatters when an inmate is killed, prompting Arthur to confront reality. In their final meeting, as Lee sings "That’s Entertainment," Arthur pleads for her to stop, signaling his desire to face the truth.