In a ruined park of Lammermoor Castle, Normanno leads a group of guards in searching for a mysterious intruder. Enrico enters and joins Normanno, disappointedly telling of his sister's refusal to marry Arturo.
Raimondo, Lucia's tutor, suggests that the loss of her mother is what is keeping Lucia from such ideas of love, yet Normanno reveals that every morning Lucia has been keeping secret trysts with a hunter who saved her from being attacked by a bull. When questioned about the identity of the man, Normanno names the family rival, Edgardo, thus infuriating Enrico ("Cruda funesta smania"). Soon guards return to confirm his suspicions, reporting that Edgardo was seen riding away from the ruined tower ("Come vinti"). Hearing the news, Enrico swears vengeance on Lucia and Edgardo, and leaves with the support of the guards and Raimondo's cries for Lucia's forgiveness unheard ("La pietade in suo favore").
As a harp interlude is heard, Lucia enters the park with her companion, Alisa, planning to meet Edgardo at a fountain near her mother's grave. Lucia tells Alisa of the maiden's ghost that haunts the fountain and once appeared to warn her of a tragic ending to her secret affair ("Regnava nel silenzio"). Despite Alisa's pleas to leave Edgardo, Lucia can think only of their love ("Quando rapita in estasi").
Soon Edgardo arrives and tells Lucia that he must leave Scotland in order to serve a political duty in France. Before departing he wishes to make amends with Enrico, sealing the agreement by asking for her hand. Knowing that her brother will not approve, Lucia begs Edgardo to keep their love secret. Though angered by Enrico's persecution, Edgardo agrees. The lovers exchange rings to seal their vows and bid each other a fond farewell ("Verranno a te sull' aure").
Several months have passed since Edgardo left Scotland. In this time, Enrico has continued with his plots to separate the lovers. His plan is now driven not only by his hatred for the Ravenswood family, but also by his impending financial need that could be aided, were Lucia married to Arturo.
In a hall in Lammermoor Castle, Enrico and Normanno discuss their plot to force the arranged marriage. They have intercepted all of the letters sent between Edgardo and Lucia during their separation, and spread a rumor that Edgardo loves another.
As Normanno leaves, Lucia joins her brother, dismayed, yet still refusing her brother's arrangement. Enrico then shows her a forged letter that indicates Edgardo's unfaithfulness to her. In shock of the news, Lucia longs for death, while her brother urges her to forget about her former love ("Soffriva nel pianto").
Festive music is heard as crowds greet Arturo outside. Enrico confesses to Lucia that if she does not marry Arturo, the family will be ruined, and he will be killed, for he has committed treasonable acts that only Arturo is powerful enough to forgive. Though he tells her that she will be to blame for his death, and that his spirit will haunt her, she continues to refuse his plan ("Se tradirmi tu potrai").
Raimondo, who also believes that Edgardo is faithless, enters as Enrico leaves, and is able to convince Lucia to marry Arturo in respect for the situation of her family ("Ah cedi, cedi"). He then assures her that earthly sacrifices will meet heavenly rewards ("Al ben de' tuoi"). The marriage is held in the great hall of Lammermoor, with many guests gathered to celebrate the union of the two families ("Per te d'immenso giubilo"). Enrico assures Arturo that his sister's sadness is due to the death of her mother, and Arturo pledges to restore the Ashton's position in society. Yet despite the supposed success of Enrico's plan, just as the marriage contract is signed, Edgardo appears, ready to claim his wife ("Chi mi frena").
Avoiding bloodshed, Raimondo orders the rivals to put away their swords. Violence is avoided, but when he sees the wedding contract, Edgardo takes the ring off of Lucia's finger, cursing her as he leaves ("Hai tradito"). Bewildered by the passing events, Lucia collapses.
Enrico finds Edgardo brooding alone in Wolf's Crag Tower and challenges him to a duel at dawn amongst the tombs of the Ravenswoods (duet: "Ah! O sole, piu ratto"). Meanwhile, the wedding celebration is disrupted by Raimondo's news that Lucia has stabbed Arturo in the bridal chamber ("Dalle stanze ove Lucia"). She enters the celebration, obviously mad and imagining herself married to Edgardo ("Il dolce suono").
Enrico enters to accuse Lucia of her crime, but is taken aback with pity for her condition. He repents, and she falls dying into Alisa's arms.
At dawn, Edgardo awaits the arrival of Enrico, and feeling betrayed by Lucia, plans to take his own life ("Fra poco a me ricovero"). Passing wedding guests inform him that the dying Lucia has called for him. Edgardo rushes to be by her side, yet Raimondo arrives to tell him that she is already dead. Realizing the steadfastness of her love, and wishing to be with her, Edgardo stabs himself and dies.
Lucia di Lammermoor