 In an episode which introduces Mildred s sister Agatha to the cast (Nancy Walker had already committed to her own series for the following season, and Agatha was to become the new housekeeper), the McMillans, Mildred and Agatha go to visit a woman relative of Mildred and Agatha, a very wealthy woman who s dying -- but not quickly enough to suit one of her nephews, who s in debt to a loansharking company later described as "just this side of Murder, Incorporated." Threatened with death if he doesn t come up with the money right away, the nephew smothers the old lady in her bed. But when the will is read, the nephew isn t the prime beneficiary -- a young great-niece is. Someone soon guns down the niece, and the nephew (who s next in line for the money) is the prime suspect. But when Agatha sneaks onto the nephew s pleasure boat to investigate him, someone plants a firebomb onto the boat. The nephew agrees to save Agatha and gives her a life jacket, but can t get off himself before the engine explodes. Next, the McMillans discover that the murdered "great-niece" was an impersonator -- so Sally poses as the great-niece herself and shows up on the doorstep of the old lady s countrified financial manager, hoping he has some clues.