SCENE 1: The entrance hall of Ravenswood Castle
Normanno urges the servants to scour the grounds for an intruder. Enrico is worried because his sister Lucia refuses to marry Arturo, an alliance which would save Enrico from the consequences of having been on the losing side in a recent uprising.
Raimondo reminds him that Lucia has not recovered from the grief of her mother's death and is not yet ready for love, but Normanno claims that Lucia is in love - with a man who had saved her from a wild bull, none other than Enrico's mortal enemy, Edgardo. Enrico's rage is exacerbated by the failure of the retainers to capture the intruder, Edgardo.
SCENE 2: The castle grounds
As Lucia and Alisa wait for Edgardo by a ruined fountain, Lucia says that she has seen the ghost of the fountain, a lady killed by her jealous lover, an earlier Ravenswood.
Edgardo announces that he is leaving at once for France on State business. Lucia refuses his request to tell Enrico of their love, rightly fearing his bitter hatred of Edgardo; and Edgardo reminds her that although he has neglected for her sake his oath to avenge his father's death on her brother, the oath still stands. She calms him and they swear eternal fidelity and exchange rings.
SCENE 1: A room in the castle
Normanno tells Enrico of the success of his scheme to intercept all letters between Lucia and Edgardo, now some months absent in France. Even though the wedding guests are already assembling, Enrico has yet to obtain Lucia's consent to the marriage, but he has a forged letter which he hopes will convince her that Edgardo plans to marry another. When she tells him that her faith is pledged to Edgardo, he overwhelms her with the letter and reminds her that only Arturo can save him from ruin. Raimondo, who has sent letters to Edgardo on Lucia's behalf, tells her that there has been no answer and advises her to sacrifice herself for her brother.
SCENE 2: The great hall of the castle
The wedding is about to be solemnised. Enrico explains Lucia's pallor and listlessness to Arturo as symptoms of her mourning for her mother. No sooner has Lucia signed the contract than Edgardo bursts in. He claims Lucia, but Raimondo shows him the contract. He throws the ring she has given him at her and demands his in return and leaves, cursing her faithlessness.
SCENE 1: The tower of Wolf's Crag
In a raging storm Enrico comes to Edgardo's home to challenge him to a duel, taunting him with the reminder that Lucia now belongs to another. They agree to fight at dawn near the tombs of the Ravenswoods.
SCENE 2: The great hall of Ravenswood Castle
The rejoicing of the wedding guests is interrupted by Raimondo, bearing the news that Lucia has gone mad and killed Arturo. Covered in blood, she enters, imagining that she is about to be married to Edgardo. Enrico's reproaches turn to remorse when he realises her state. Her wandering mind becomes more disturbed as she remembers Edgardo's anger, and she collapses.
SCENE 3: By the tombs of the Ravenswoods
Waiting for dawn by the tombs of his ancestors, Edgardo thinks bitterly of Lucia's apparent faithlessness. Tidings of her imminent death are followed by the death knell. He realises that he has misjudged her and stabs himself, hoping to join her in death.