Married couple Nell and Simon host a traditional Christmas dinner each year at their charming country estate, inviting their former schoolmates along with their partners. This year's gathering is particularly noteworthy, as the guests don formal attire and the children are given the unusual freedom to use profanity. However, beneath the festive atmosphere lies a grim reality — a looming environmental disaster, where a devastating gas cloud threatens to annihilate most forms of life. In response to this crisis, the British government has distributed suicide pills, offering a swift exit before the catastrophe reaches their shores.
As the evening unfolds, it becomes clear that Nell, Simon, and their circle of friends have made a harrowing pact: to take the pills and administer them to their children. Among the group, James is on board with the decision, but his young wife Sophie is grappling with the news of her pregnancy, leaving her in a state of uncertainty about whether she should take the pill. In a poignant moment, Nell, Simon, and their children set up a video call with Nell’s mother, currently abroad, to bid farewell, which leaves their eldest son, Art, deeply troubled.
As Art starts to question the validity of the scientists’ claims regarding the impending doom, he confides in James and Sophie about his doubts. Sophie admits her dread of the poison but expresses her reluctance to take the pill. This leads Art to declare his wish to remain with Sophie, grasping her hand until the end. Faced with the impossible situation, Nell and Simon try to persuade Art to stick to their plan and take the pill, but he vehemently rejects their pleas and flees. In a chilling discovery, he stumbles upon a deceased family by the roadside, a stark reminder that the poison is already encroaching.
Upon returning home, the group realizes that the clock has struck midnight, signaling the moment to take their pills. They retreat to their rooms, preparing for their final farewells. Just before consuming the pills, calamity strikes when Art, who had consumed some of the poison earlier, succumbs to its effects. James urgently tries to convince Sophie to follow through with the pact, emphasizing that he will only do so if she does as well. The morning reveals a chilling scene: everyone is gone, save for Art, who inexplicably opens his eyes. The weight of their choices hangs heavy, leaving behind a legacy of despair and unresolved emotions as the shadows of their fateful decision linger in the air.
