Justine, a newlywed struggling with deep-seated melancholia, arrives with her husband, Michael, at her sister Claire’s lavish estate to celebrate their wedding reception. Despite the opulence and the efforts of her wealthy brother-in-law, John, to organize a perfect evening, Justine is increasingly consumed by sadness and detachment, finding herself at odds with the expectations of those around her. As the night progresses, her mental state deteriorates, affecting her relationships and casting a shadow over the festivities. Her professional life, personal expectations, and the semblance of happiness she tries to project begin to unravel amidst the tension of artificial social niceties and her own inner turmoil.

Meanwhile, an astronomical anomaly looms as a rogue planet named Melancholia is discovered to be heading towards Earth, inciting varying degrees of concern, denial, and fascination among different people, including Justine and Claire. While Claire becomes increasingly anxious about the impending celestial event, Justine shows a peculiar calmness in the face of potential disaster. As the menacing planet draws closer, the dynamics of the sisters' relationship, their perspectives on life, and their coping mechanisms come into sharp focus against the backdrop of the impending cosmic collision, leading to vastly different emotional reactions and existential contemplations of their place in the universe.