"Lucia di Lammermoor" is set not in Italy's fair Verona, but in Scotland's craggy highlands. On the moors there are also two households, both alike in dignity, the Ashtons and the Ravenswoods. Before the opera begins, Lucy (Lucia) Ashton, the sister of Henry (Enrico), the head of the Ashton Clan, has fallen in love with Edgar (Edgardo) Ravenswood, the last of the Ravenswoods.  When Enrico discovers this love, he is mightily displeased, first because he bears all his family's ancient enmity for the Ravenswoods and second because it is highly inconvenient for him politically. He has been an ardent backer of the faction supporting William III, the Dutch-born king who has shared the throne of Great Britain with his wife Mary, the head of an opposing faction. But now William is dead and Mary is the sole monarch of the realm. Enrico's enemies are making unpleasant plans for both him and his clan. To save himself, Enrico as engineered a political alliance with the rich and powerful Arthur (Arturo) Bucklaw, but this alliance is based on the marriage of Bucklaw to Lucia.

Lucia and Enrico secretly meet in the gardens of the Ashton estate. Edgardo informs Lucia that he must leave Scotland for a time and travel on a mission to France. Before he leaves, Edgardo gives Lucia a ring to symbolize their undying love for one another.

Time passes and Lucia hears nothing from Edgardo. Behind her back, Enrico and his steward Norman (Normanno) have conspired to intercept all of Edgardo's letters and have even forged documents saying that Edgardo has taken up with another woman in France.

On the day of the wedding to Arturo Buklaw, Enrico browbeats his sister into believing that Edgardo has abandoned her and that she has no alternative to marrying Bucklaw. The Ashton family chaplain, Raymond (Raimondo) Bide-the-Bent, although not in the conspiracy, also tries to convince Lucia that she should marry Bucklaw.

The wedding party begins with all the Ashton clan gathered to celebrate. Enrico brings in Arturo Bucklaw, the great grandee t the joy of all. Arturo pledges that he will be Enrico's brother, friend and defender. Lucia arrives, pale and unhappy. Enrico explains to the puzzled Arturo that his sister is still grieving the recent death of her mother. Enrico quietly berates Lucia for her lack of caution and insists that she sign the wedding contract. Reluctantly she does so, saying that she has signed her own death warrant.

At that moment, Edgardo appears, returned at last from France and ready to claim his true love. Enrico and Arturo challenge him. Raimondo explains that Lucia is married to Arturo. He shows Edgardo the signed contract. Edgardo demands of Lucia whether the signature on the paper is actually hers. She whispers, "Yes." Enraged, Edgardo tears his ring from Lucia's hand, tosses it to the floor and curses her faithlessness. Enrico, Arturo and all the Ashtons vent their rage on Edgardo and drive him away.

Later, after the newlyweds have departed to their private chambers, the Ashtons are still celebrating. Suddenly Raimondo appears. He has heard strange cries from the rooms of the just-married pair. Inside he has discovered the dead body of Arturo on the wedding bed and Lucia, bloodstained and apparently mad, holding a dagger.

Lucia enters in her red-spattered nightgown, dagger in hand. She sings erratically,of her love for Edgardo, of her betrayal by her brother, of Edgardo's denunciation and rejection and of poor Arturo. She sings and sings until she collapses into the arms of her appalled brother.

Later, Edgardo reflects upon Lucia's betrayal of his love and her infidelity, but despite everything, he still loves her as passionately as ever. He hears the sound of a funeral procession. He asks who is to be buried. It is Lucia. It is too much. All is gone for him save his love for his lost Lucia. He stabs himself and dies in grief. Raimondo and the mourners call on heaven to forgive him.