Among the Lammermoor Hills of Scotland, Normanno and the castle guards search for a prowler spotted on the grounds of Ravenswood Castle. The lord of the castle, Enrico Ashton, in a state of agitation, tells Normanno that he must escape financial ruin by arranging a wealthy husband for his sister Lucia, but that she has so far refused. The chaplain Raimondo assures him that the girl’s refusal to wed stems from her grief after the recent death of her mother, but Normanno reveals the truth: Lucia has fallen in love with Edgardo, the heir of the former lord of Ravenswood, whose estates and title Enrico has usurped. The guards return to report that Edgardo was seen trespassing on the premises.
Lucia tells her companion Alisa that she has been meeting Edgardo in secret at the well on the castle grounds. It is a place that fills Lucia with dread, for she has seen the ghost of a murdered woman there. Alisa urges her to give up this ill-omened affair, but Lucia swears that Edgardo is her only source of happiness. When Edgardo arrives, he tells her that he must travel to France on a diplomatic mission; he hopes to make peace with her brother and ask him for her hand in marriage before he embarks. Knowing the depths of her brother’s hatred, Lucia begs him not to do so. They exchange rings, vowing eternal love for one another.
During Edgardo’s absence, Enrico has intercepted all of his enemy’s letters to Lucia. He has also gathered his kinsmen for Lucia’s wedding to Lord Arturo Bucklaw, whose arrival is imminent. Enrico shows Lucia a forged letter that deceitfully states Edgardo’s love for another woman. Enrico tells Lucia that she will marry Arturo that very day.
Raimondo, invoking the memory of her mother, urges Lucia to go through with the ceremony for the good of the family. She is too stunned to object and he reminds her that there are heavenly rewards for earthly sacrifices.
In the great hall of the castle, guests hail the union of two important families. Arturo pledges to restore the Ashtons’ prestige. Enrico prepares him for Lucia’s melancholy by explaining that she is still in mourning for her mother. As soon as Lucia signs the marriage contract, Edgardo enters, to the astonishment of the wedding guests. Enraged that Lucia has apparently broken her vows to him, he prepares to fight; Raimondo is able to restrain him. Edgardo furiously demands his ring from Lucia, cursing her.
Edgardo returns to his destitute home, the tower of Wolf’s Crag. Enrico follows him there, challenging him to a duel for insulting his sister and his family’s honor. Edgardo accepts, eager to avenge his father’s murder at the hands of Enrico. They agree to fight at dawn near the graves of Edgardo’s ancestors.
Meanwhile, the wedding celebrations at the castle continue. Raimondo bursts in with the shocking news that Lucia has killed her new husband. She enters, bloodied by the deed and irreparably altered. When Enrico storms in, he is silenced by the sight of her pitiful condition. Believing herself in heaven, Lucia falls dying. As she is carried away, Raimondo turns on Normanno, blaming him for the misfortune that has fallen on the family.
At the tomb of the Ravenswoods, Edgardo laments Lucia’s supposed betrayal and awaits his duel with Enrico, which he hopes will end his own life. He learns of Lucia’s madness and her request to see him once more on her deathbed. As he is about to rush to her side, Raimondo arrives with news of her death. Unable to bear the thought of life without her, Edgardo kills himself.