It's about a novelist (Vida) who hires another writer to write her biography. She wrote 12 stories about her own life but they were all different because she was lying, so the last one was called the 13th tale lol. She told her about twins named Adeline and Emmeline, Emmeline was good, Adeline was evil. Their mother went to a mental hospital when they were 9, their father was their mother's brother and he was found dead in the woods. So they were taken care of by their maid and their gardener. Their family doctor hired a "governess" to teach them, Emmeline listened to her and did what she told them but Adeline didn't so they separated them and left Emmeline at home and took Adeline to the doctor's house. While Adeline was with the doctor the governess saw Emmeline out in the woods with Adeline so she ran over to the doctor's house to tell him that she didn't want them together but Adeline was still in the house in bed. The governess quit because she thought she was going crazy. When the girls were 17 their maid died, she fell off the stairs. Then their gardener was killed, someone broke the safety latch on his ladder and he fell. Vida said it wasn't Emmeline and she didn't do it.
So the writer was trying to figure out who was killing everyone If Emmeline wasn't doing it and Adeline (supposedly Vida) wasn't doing it.