Mrs McCarthy's friend, Freda, dies in her sleep at a care home. Father Brown suspects foul play when he smells an unusual odour from the body. Animosity between Freda's son and Freda's new friend, Charlie, who is to inherit Freda's estate arouses Father Brown's suspicions until Charlie dies in his sleep. Another death and Mallory arrests Freda's son. All the victims were terminally ill and a white feather at each scene confirmed they were complicit in their own death by an angel of mercy. Bunty's friend, terminally ill Ellen, wheelchair bound, goes missing and she fears the worst after finding a white feather in her room.