Sylvia Savage is a non-canonical character created for the Agatha Christie's Marple adaptation Why Didn't They Ask Evans? She is derived from Sylvia Bassington-ffrench, a character in the original novel but has a different backstory and a different role in the plot. Sylvia Savage is portrayed by Samantha Bond. A younger Sylvia Savage, seen only in flashbacks, is portrayed by Sarah Ridgeway.
Unlike Sylvia Bassington-ffrench, Sylvia Savage is not an American heiress. In this adaptation the family trees of the Bassington-ffrenches and the Savages are combined. At the time of the events in the episode, Sylvia Savage lives at Castle Savage (a rough equivalent to Merroway Court) where most of the events take place.
Many years before the events in the episode, Sylvia was married to George Savage, one of two brothers in the Savage family. However, Sylvia later met the brother Jack. While all of them were in China, Sylvia had an affair with Jack. Together they plotted and killed George and then married each other. With the war coming, Jack left China with Sylvia, and forced her to leave behind and abandon her two children from her first marriage, Alice and Michael.
Years later, while Sylvia and most of the household were on a holiday on the Isle of Wight, Jack Savage, who had been left behind at the family home at Castle Savage, died, apparently of a cardiac arrest.
Jack left behind a strange will, disinheriting his family and leaving everything to the Quang Ho orphanage in Shanghai, China. John Carstairs, who had known Jack in China believed this was out of character and started investigating but was murdered.
In pursuing the trail of the crime, Miss Marple, Bobby Attfield and Frankie Derwent found that Alice and Michael had come back as Moira Nicholson and Roger Bassington respectively. They plotted to lure the household away from Castle Savage, fake a new will and then murder Jack Savage. Miss Marple and the others pieced the puzzle together just in time to surprise Moira and Roger in the final act of trying to kill Sylvia by injecting her with snake venom.
Nonetheless, Sylvia admits to Miss Marple that the provisions of the new will (though now exposed as a fake) wouldn't have mattered. She wants all the money to go to the children and has no wish to benefit from the wealth of her dark history. In the interim, Sylvia had become a drug addict, being regularly plied with drugs by Dr Alec Nicholson. She says the drugs were needed to ease the pain (probably mental) for what she had done.