By the Bog of Cats centers around the character of Hester Swane. The play opens with Hester carrying a dead swan through the snow to bury it. The Ghost Fancier is watching her and she tells him that she used to play with the swan when she was young. The Ghost Fancier, realizing that Hester is still alive, decides that he will have to come and collect her at dusk—implying that she will die. Monica, Hester's friend, enters and tells her that she must stop her brooding, or she will lose her child, Josie. Caroline Cassidy now owns the property and is telling Hester to leave. Caroline is also marrying Carthage, the father of Hester’s child. Carthage built the house for Hester. However, Hester refuses to give up her home or Carthage without a fight.
Hester meets with the Catwoman, a blind woman in her fifties who wears a coat of cats. They talk about Hester’s mother, Big Josie Swane, and the fact that she abandoned Hester when Hester was seven years old. Catwoman tells her that her mother was not a good person and that she should not wait for her to return, and that Josie placed her in the black swan’s lair and said the child would live as long as the black swan, and not a day longer.
Caroline Cassidy goes to visit Hester and tells her that she needs to leave, but Hester does not hear her and threatens Caroline. Hester then meets with Carthage and begs him not to leave her. She reminds him that he got the money for the farm from her, and says that she will do anything to keep him. Carthage says he is marrying Caroline and that her father is signing over his farm to him that evening. He tells her to be out of the house by dusk and gives her an envelope of money, but she refuses it.
Xavier Cassidy, Caroline's father, also visits Hester and tells her to leave, but she continues to refuse. The ghost of Joseph, Hester's brother who she murdered, visits and tries to tie up loose ends. Hester crashes Caroline and Carthage's wedding, wearing her own wedding dress. When they force her to leave, she threatens revenge.
In the final act, Hester has lit Carthage's farm on fire. Hester speaks with her dead brother, Joseph, and tells him that she did not kill him for money, but because she was jealous of the love that their mother showed to him. After this, Xavier Cassidy comes to try and get Hester to leave, and sexually assaults her, suggesting that her mother was a whore. When he leaves, Carthage comes on, and Hester confronts him about his role in her brother's death. This causes him to threaten to take Josie, her daughter, away from her for good.
Left alone, Hester goes to slit her own throat, but is interrupted by little Josie, who wants to stay with her mother. In a fit of madness, Hester slits Josie's throat. The Ghost Fancier comes and kills Hester.