At an elegant countryside estate, Nell and Simon host a lavish Christmas gathering for their closest friends, each arriving with a mix of anticipation and underlying tension. The group includes their longtime pal Sandra and her new partner Tony, as well as Bella with her feisty fiancé Alex. The festive atmosphere is shadowed by a pervasive dread; the reality of an impending environmental catastrophe hangs over their celebration. This apocalyptic event, foretold to be lethal, has driven the British government to issue suicide pills to its citizens. The group struggles to balance the holiday's joy with the ominous undertone of their final night.

As the evening unfolds, conversations oscillate between nostalgic reminiscence and anxious debates about their looming fate. The children, including Nell and Simon's precocious son Art, are mostly kept in the dark about the gravity of the situation, amplifying the adults' need to put on a brave face. As midnight approaches, each character faces their mortality in unique ways, revealing deep-seated fears, long-buried secrets, and declarations of love and regret. Conflicting emotions bubble to the surface, testing relationships and questioning their acceptance of destiny versus the instinct to survive. Amidst laughter, tears, and poignant memories, the group’s humanity is laid bare against an uncertain future.