Sister Agnieszka is a non-canonical character created for the 2008 ITV adaption of Appointment with Death. She is a Polish nun who accompanies the tour of the Boynton family to the archaeological excavation at Ain Musa. Sister Agnieszka is portrayed by Beth Goddard in her second appearance on the series. She had formerly portrayed Violet Wilson in 1993.
Dame Celia Westholme, who joined the tour halfway, told Poirot that Poirot that she found Sister Agnieszka very suspicious because she always seemed to be around Jinny, the youngest of the Boynton children. Later during the show, Sister Agnieszka was injured in the head in Jinny's tent. According to the events as reconstructed from the account of Jinny, it appears that somebody had tried to kidnap Jinny by placing a hood over her head. She had grabbed a rock and struck at the assailant. It appeared to everyone that Sister Agnieszka had responded to her cries for help and had tried to intervene but had been struck by accident.
During the denouement, however, Poirot revealed that Sister Agnieszka was in fact an agent working for a gang of slavers. She was the one who was trying to put a hood over Jinny's head. The sister escaped from Ain Musa by stealing a car and driving off into the desert but this ran out of water and overheated. She tried to continue the journey on foot but is overcome by the heat and thirst and died.
Whether or not she is a real nun is a matter for conjecture. In one scene, she quotes from the King James version of the Bible, which is not a Catholic version. However as she was talking to Jinny, she might well have chosen a version she expected Jinny to be familiar with. In another scene, she is seen on her own praying before an altar, which again, is an unusual action for someone who is actually a criminal and who only adopts a disguise as a nun.