Act I, Scene I: The grounds of Ravenswood Castle
Normanno, captain of the guard at Lammermoor Castle, sends his men to discover the identity of the man that Lord Enrico Ashton’s sister, Lucia, has been meeting in secret. Enrico reveals that his family’s power has weakened due to a political change, and he needs Lucia to marry a well-to-do nobleman, Arturo, in order to save and protect his clan. But Lucia refuses. Raimondo, the family chaplain, says it is because Lucia’s and Enrico’s mother has recently died. But Normanno tells Enrico that Lucia is madly in love with a man he suspects is Edgardo Ravenswood, Enrico’s hereditary enemy. This infuriates Enrico, and when Normanno’s men return with the news that Lucia’s mysterious lover is indeed Edgardo, Enrico in a rage swears revenge on the lovers.
 

Act I, Scene II: A garden outside Ravenswood Castle
Lucia and her companion, Alisa, wait for Edgardo in the woods. Nearby is an ancient fountain. Lucia tells Alisa that she had once seen the ghost of a young woman who was slain long ago by her jealous lover, a Ravenswood, and was left forever submerged in the waters. The ghost beckoned to Lucia from the blood-red waters of the fountain. Alisa fears it is an evil omen and begs Lucia to forget Edgardo. At the thought of Edgardo, Lucia’s attitude changes to one of elation. Edgardo arrives and tells Lucia he must go to France in the interests of Scotland. Before leaving, he wishes to reconcile himself with Enrico and ask him for her hand in marriage. Terrified of her brother, she begs him to keep their love secret. Edgardo is reminded of his rage towards Lucia’s family—but his love for her calms his fury. They exchange rings and Celtic marriage vows and promise to write during their separation.

Act II, Scene I: Enrico’s chambers
Enrico has arranged the wedding between Arturo and Lucia, but Lucia has not agreed. The guests are arriving. Normanno and Enrico have been intercepting Edgardo’s letters to Lucia. Normanno gives Enrico a letter he has forged in Edgardo’s hand telling Lucia he loves another. Enrico tries to reason with Lucia to marry Arturo. Lucia upbraids Enrico for his harsh treatment of her, and she is adamant in her love for Edgardo and says she is already married. Enrico shows her the forged letter. Lucia is devastated, believing Edgardo no longer loves her. Enrico comforts her, begs her to save him by marrying Arturo, and finally threatens her with nightmares of his own death. Crushed and exhausted at Edgardo’s alleged faithlessness, she agrees to marry Arturo.

Act II, Scene II: The Great Hall at Lammermoor Castle
The wedding guests have gathered and Arturo extends his hand in friendship and loyalty to Enrico. Enrico explains to Arturo that Lucia seems listless because she is still mourning the death of her mother. Arturo asks about a rumor of Lucia and Edgardo. Lucia enters, saving Enrico from having to answer. Lucia unwillingly signs the marriage contract. Edgardo forces his way in, demanding an explanation. All are about to draw swords when suddenly their sympathy for Lucia overwhelms them and for a moment their anger subsides. But when Edgardo  sees Lucia’s signature on the contract he curses her and her family, tears his ring from her finger, and leaves.

Act III, Scene I: Later in the Great Hall
The wedding festivities are in progress when the chaplain, Raimondo, stops the proceedings and tells the guests that Lucia has gone mad and has murdered Arturo in their nuptial bedroom. Lucia appears, smeared with blood and carrying Arturo’s dagger. Delirious, she imagines herself being in a wedding ceremony with Edgardo—and how heavenly it is! Enrico enters and Lucia, in her insanity, mistakes him for Edgardo, declaring herself to be his once more. Enrico is remorseful. Lucia, still believing Enrico to be Edgardo, tells him that she will pray for him from heaven and will not be happy until he joins her there.

Act III, Scene II: The Ravenswood graveyard later that night
Edgardo has been challenged to a duel by Enrico. Edgardo has chosen to die by Enrico’s sword. Believing Lucia is married to another, he no longer wishes to live. A group of wedding guests passes by and tells him that Lucia is mortally ill and that she calls for him. A death knell sounds. As Edgardo rushes towards the castle, he is stopped by Raimondo. Lucia is dead. Edgardo sees Lucia in heaven—a sweet, loving presence looking down serenely. Drawing his dagger, he goes to be with her. As Raimondo calls to God to forgive this dreadful deed, the dying Edgardo thinks only of Lucia.