The play opens on a late Sunday evening in the parlor of the Borden house in 1902, in Fall River, Massachusetts. Miss Lizzie enters with tea for the Actress, a friend who is perhaps more than a friend. The Actress tells Lizzie that she has heard children singing a song about how Lizzie axed her parents to death. The Actress asks Lizzie if the rumors are true, and Lizzie decides that they should act out the evening, with the Actress playing Lizzie and Lizzie playing the Irish maid, Bridget.
In a flashback to the Borden family home, we meet Wingate, Lizzie's step-uncle, who is often sexually inappropriate with her. Lizzie's stepmother, Mrs. Borden, is scheming with Mr. Borden to acquire more money and land from the patriarch, which would leave Lizzie and her meek sister, Emma, as their dependents.
They act out the flashback, in which we learn that Lizzie loves her father, but feels that he is mean to her and falls prey to the machinations of her stepmother, Mrs. Borden. Mr. Borden wants Lizzie to marry, but she has no interest in marriage and has always felt that she never fit in with the expectations of her family members, who want her to be a society lady. She simply wants independence and to take care of the pigeons that she keeps in the shed out back.
We also learn that Lizzie is friends with Dr. Patrick, a married Irish doctor in town, but that this has led to a lot of speculation about the nature of their relationship. Lizzie insists that they are simply friends, but her father tries to talk her into marrying someone more available, in spite of Lizzie's protestations. Trying to curb Lizzie's rebelliousness, Mr. Borden hits her and kills all of her pigeons, chopping their heads off in front of her. He also plots to sign away the farm and the mill house to Wingate, Mrs. Borden's brother, thus taking away Lizzie's inheritance.
Driven almost mad by this injustice, Lizzie becomes intent on doling out her own justice to her stepmother, whom she believes to be the root cause of her suffering. One day, while she is home alone with Mrs. Borden, Lizzie kills her with a hatchet. While she is trying to hatch a plot to cover it up with Bridget, the maid, Mr. Borden unexpectedly comes home, and Lizzie murders him too.
Back in the present, the Actress believes that Lizzie did kill her parents. Lizzie's sister, Emma, comes down the stairs complaining about the noise of Lizzie's reenactment. She forbids Lizzie from seeing the Actress and asks her sister yet again if she actually killed their parents. Lizzie tells her that, if she did kill their parents, then Emma is partially responsible, since she raised Lizzie. The Actress tries to intervene, reminding Lizzie that she is the one who killed the Bordens, but Lizzie informs the Actress that she, the Actress, is the one who did it.