Set over a single day and night, “Late September” unfolds in a picturesque house and garden in Kent. The story revolves around a birthday party thrown by a middle-aged woman for her husband, marking nearly four decades of marriage. As the celebration progresses, long-standing conflicts, budding relationships, and hidden truths come to light among the friends gathered. The simmering tensions within the couple’s marriage also begin to surface, unable to remain hidden any longer.