The film follows the tumultuous and destructive relationship between Sid Vicious, bassist of the notorious punk rock band the Sex Pistols, and Nancy Spungen, an American groupie. Meeting during the height of the punk rock movement in London, their intense connection quickly spirals into a codependent and chaotic affair fueled by drug addiction. Amidst the backdrop of the band's disintegration and the chaotic punk scene, Sid and Nancy's relationship becomes increasingly toxic, marked by passionate highs and devastating lows. Their time together is a whirlwind of music, violence, and substance abuse, with their bonded self-destructive tendencies pushing them further into turmoil.

As the Sex Pistols embark on a disastrous tour of America, Sid's behavior becomes more erratic, influenced heavily by his dependence on drugs and his volatile relationship with Nancy. Following the band's eventual collapse, Sid attempts to launch a solo career but struggles with his own demons and legal troubles. With their lives spiraling out of control, Sid and Nancy retreat into a life isolated from reality, consumed by their addiction and each other. Their story serves as a gritty, unflinching depiction of love and self-destruction set against the anarchic landscape of the late 1970s punk rock era.