Scotland, mid-19th century. An intruder has been spotted at night on the grounds of Lammermoor Castle, home of Enrico Ashton. Normanno, the captain of the guard, sends Enrico’s men off in search of the stranger. Enrico arrives, troubled. His family’s fortunes are in danger, and only the arranged marriage of his sister, Lucia, with Lord Arturo can save them. The chaplain Raimondo, Lucia’s tutor, reminds Enrico that the girl is still mourning the death of her mother. But Normanno reveals that Lucia is concealing a great love for Edgardo di Ravenswood, leader of the Ashtons’ political enemies. Enrico is furious and swears vengeance. The men return and explain that they have seen and identified the intruder as Edgardo. Enrico’s fury increases.
Just before dawn at a fountain in the woods nearby, Lucia and her companion Alisa are waiting for Edgardo. Lucia relates that, at the fountain, she has seen the ghost of a girl who was stabbed by her jealous lover. Alisa urges her to leave Edgardo, but Lucia insists that her love for Edgardo brings her great joy and may overcome all. Edgardo arrives and explains that he must go to France on a political mission. Before he leaves he wants to make peace with Enrico. Lucia, however, asks Edgardo to keep their love a secret. Edgardo agrees, and they exchange rings and vows of devotion.
It is some months later, the day on which Lucia is to marry Arturo. Normanno assures Enrico that he has successfully intercepted all correspondence between the lovers and has in addition procured a forged letter, supposedly from Edgardo, that indicates he is involved with another woman. As the captain goes off to welcome the groom, Lucia enters, continuing to defy her brother. Enrico shows her the forged letter. Lucia is heartbroken, but Enrico insists that she marry Arturo to save the family. He leaves, and Raimondo, convinced no hope remains for Lucia’s love, reminds her of her dead mother and urges her to do a sister’s duty. She finally agrees.
As the wedding guests arrive, Enrico explains to Arturo that Lucia is still in a state of melancholy because of her mother’s death. The girl enters and reluctantly signs the marriage contract. Suddenly Edgardo bursts in, claiming his bride, and the entire company is overcome by shock (Sextet: “Chi mi frena in tal momento”). Arturo and Enrico order Edgardo to leave but he insists that he and Lucia are engaged. When Raimondo shows him the contract with Lucia’s signature, Edgardo curses her and tears his ring from her finger before finally leaving in despair and rage.
Enrico visits Edgardo at his dilapidated home and taunts him with the news that Lucia and Arturo have just been married. The two men agree to meet at dawn by the tombs of the Ravenswoods for a duel.
Back at Lammermoor, Raimondo interrupts the wedding festivities with the news that Lucia has gone mad and killed Arturo. Lucia enters, covered in blood. Moving between tenderness, joy, and terror, she recalls her meetings with Edgardo and imagines she is with him on their wedding night. She vows she will never be happy in heaven without her lover and that she will see him there. When Enrico returns, he is enraged at Lucia’s behavior, but soon realizes that she has lost her senses. After a confused and violent exchange with her brother, Lucia collapses.
At the graveyard, Edgardo laments that he has to live without Lucia and awaits his duel with Enrico, which he hopes will end his own life. Guests coming from Lammermoor Castle tell him that the dying Lucia has called his name. As he is about to rush to her, Raimondo announces that she has died. Determined to join Lucia in heaven, Edgardo stabs himself.