Beautiful, wealthy Rosemary Barton collapses and dies at her own birthday dinner in a fashionable London restaurant, cyanide in her champagne. The inquest records suicide, attributing it to depression following influenza, and the people seated around the table return to their lives. A year later, Rosemary's widower, George, receives anonymous letters insisting that his wife was murdered. Convinced and increasingly obsessed, he resolves to gather the same six guests for a second dinner staged in her memory, hoping the reconstruction will force the truth into the open. Each of the surviving guests carries a private reason to have wished Rosemary gone, and the early chapters move through their memories in turn, revealing a web of love affairs, jealousy, money, and ambition that the dead woman left behind. When the memorial dinner ends in a fresh tragedy, Colonel Race, a friend of the family with a background in intelligence work, joins Chief Inspector Kemp to untangle who sat at the table with the intent to kill and how the poison was delivered without anyone noticing. The investigation tests every alibi and motive, weighing the rising politician, the watchful wife, the devoted secretary, the secretive admirer, and the grieving younger sister against one another. Built from shifting perspectives that circle the same evening, the story keeps its solution balanced on small observations until the final pages.