Biographer Margaret Lea (Olivia Colman) arrives at the country house of famous novelist Vida Winter (Vanessa Redgrave). She has been invited to stay there and help Vida write her biography before she dies of cancer.
Margaret is hesitant, as Vida is known for telling a different story each time she is asked about her background in interviews, so she requests some verifiable information from public record.
Vida reveals her birth name was Adeline March and the local newspapers wrote about a fire that burned down her family home when she was seventeen, of which she bears proof in the form of a key-shaped burn on her palm.
Vida tells her the events leading up to the fire. She grew up at Angelfield, the decaying family estate, with her identical twin sister Emmeline. Their mother Isabelle was suffered abuse at the hands of her unhinged brother, Charlie, and eventually taken away to a mental asylum, so the girls were mostly left to their own devices, becoming unruly, anti-social and running wild.
The only adult supervision they had came from the two servants, nicknamed "Missus" and "John-the-dig." Ambrose, a young man who is the hired help gets Emmaline pregnant. This leads to Adeline trying to kill the baby.
Margaret works out there were three girls at Angelfield. Vida confirms this, revealing she wasn't Adeline, but presumably the daughter of Charlie, abandoned at the estate by her unknown mother. The real Adeline was dangerously violent and jealous of anyone who got Emmeline's attention. This means there three orphans at the house. Margaret reveals her story and admits that she herself had a twin who was fatally struck by a car as a child, a tragic accident for which Margaret has always blamed herself.