At the beginning of the play, Hester is dragging a dead black swan across the snow and ice at the Bog of Cats. The Ghost Fancier has come to collect her but realizes that he is early as Hester is still alive. The Ghost Fancier exits and says that he will return at a later time. Hester’s neighbour, Monica, enters and attempts to convince Hester to accept the impending marriage between Hester’s former lover, Carthage, and his much younger bride, Caroline Cassidy. Upon this marriage, Carthage will inherit Cassidy’s father’s farm. While digging a grave for the swan, Hester talks with the Catwoman about how her mother, Josie Swane, left her in the swan’s lair as an infant and claimed: “that child…will live as long as this black swan, not a day more, not a day less." During this conversation, the audience learns that Hester has been waiting on the Bog of Cats for her mother, Josie, since she was abandoned at age 7. The Catwoman urges Hester not to wait any longer for her mother, but Hester refuses to leave the Bog. In the next scene, Hester and Carthage's 7-year-old daughter Josie plays cards and discusses Carthage's upcoming wedding with her grandmother, Mrs Kilbride. Mrs Kilbride cheats her granddaughter at cards and talks disparagingly of Hester because of Hester's lack of fortune and inferior “tinker” blood. Meanwhile, Caroline Cassidy visits Hester in her wedding dress and attempts to convince Hester to allow her to marry Carthage without disruption and to leave the house on the bog, which Hester has previously signed over to Carthage. Hester scorns Caroline and reminds her that Hester was once her babysitter. Hester challenges Caroline about Carthage and claims: “Carthage Kilbride is mine and only mine. He’s been mine since he was sixteen. You think ya can take him from me? Wrong. All wrong. Now get out of me sight." Following Caroline's visit, Carthage also visits Hester in his wedding clothes and also attempts to convince her to leave the house. Hester reminds him that it was with her money that he bought his first land and accuses him of “sellin’ me and Josie down the river for a few lumpy auld acres and notions of respectability." Xavier Cassidy, Caroline’s father, also visits Hester to urge her to leave. Hester refuses to listen to any of the visitors and insists that she is staying on the Bog of Cats until her mother returns for her.
This act depicts the marriage of Caroline Cassidy and Carthage Kilbride, which is disrupted by various individuals, both real persons and ghosts. The ghost of Joseph Swane, Hester’s murdered brother, returns and talks to the Catwoman and Hester to find out why Hester killed him. Father Willow, the officiant, and the Catwoman talk of going on a vacation together. Mrs Kilbride, Carthage’s mother, wears a white dress to the wedding and poses in pictures with her son like a bride and groom. She also interrupts the bride’s father’s toast in order to make her own speech about her son. To scorn Caroline and Carthage’s insult of coming to her house in their wedding clothes, Hester appears at the wedding in a wedding dress that Carthage had bought for her nine years ago, along with promises to marry her. Mrs Kilbride and Xavier Cassidy attempt to violently eject Hester from the wedding, while Hester’s neighbour Monica chides them for being too hard on Hester. Hester lashes out in anger against the wedding guests and pleads with them, insisting: “I can’t lave till me mother comes. I’d hope she’d have come before now and it wouldn’t come to this. Don’t make me lave this place or somethin’ terrible’ll happen." When she is finally forced to leave the wedding, Hester swears revenge and yells: “You’re lavin’ me no choice but a vicious war against ya." 
The final act is characterized by death and destruction. As her house blazes behind her, Hester talks with the ghost of her brother, Joseph Swane, and explains that she did not kill him for money; rather, she envied his relationship with their mother. Hester informs him that Carthage took Joseph’s money, used it to buy land for himself, and left Hester for another woman. Hester’s neighbour, Monica, interrupts this conversation to tell Hester that her house is on fire. Hester, having set the fire herself, is completely indifferent. Monica begs Hester to leave, and Hester insists that she is still waiting for her mother. Xavier Cassidy comes to chide Hester for ruining the wedding. Shortly after Xavier, Carthage arrives and is outraged to see the fire destroying the home and the livestock. Hester angrily accosts Carthage for his role in her brother’s death. Carthage swears that he will take Josie away from Hester by having her declared an unfit mother. Due to the Ghost Fancier’s visit, Hester knows that she is fated to die. When Josie begs Hester not to leave her, Hester slits her throat in order to spare her the same fate of living without her mother. The Ghost Fancier performs a slow death dance with Hester before plunging a blade into her chest, killing her.