A theatre director and his wife bring together a group of performers in an isolated, mountainous part of southern France.

Late September takes place over a 24-hour period, in a beautiful Kent house and garden, and follows the course and aftermath of a birthday celebration organised by a middle-aged woman for her husband to whom she has been married for nearly 40 years. As the day and night progress, old rifts, new relationships and secrets emerge amongst friends, and the underlying tensions in the marriage can no longer be contained.