Following the death of Rosemary Barton, her husband receives an anonymous letter saying she was murdered. George Barton decides to repeat the dinner occasion. During the meal, they raise a glass of champagne to toast Iris, (Rosemary’s sister), but Iris does not drink. They all leave the table to dance. When leaving the table Iris’s bag falls to the floor, which is replaced on the table, by a waiter, in the wrong place. When the guests return to the table they all sit one place round, out of order. At this point, George drinks from the poisoned glass and dies.
If Iris dies her money would go to Lucilla Drake. Her son, Victor Drake, works with Ruth Lessing (George Barton's secretary) to set up the scene. Dressed as a waiter, Victor puts the poison into Iris’s glass. However, as they have all moved around by one seat, it is George Barton that dies. In a second attempt, Ruth tries to run over Iris in a car, and thirdly, she tries to gas her after knocking her unconscious.
Hope that helps. It's an expansion of the short story, Yellow Iris.