The scene is set in Scotland in the eighteenth century.

Scene I: A forest. While out searching for a trespasser with his men, Enrico learns that his sister Lucia has been meeting secretly with his enemy Edgardo. Their families have long been feuding. Enrico is enraged by the meetings and swears revenge.

Scene 2: A fountain. While waiting for a rendezvous with Edgardo, Lucia tells her lady Alissa of a ghost that haunts the fountain and prophesies a tragic end to the romance between Edgardo and herself. When Edgardo comes, it is with news that a mission is going to take him away to France. He wants to marry Lucia and end the feud between their families to do so, but Lucia begs him to keep their love a secret. He agrees. Enrico has persecuted Edgardo for too long ever to permit him to wed Lucia. Edgardo and Lucia swear their love and separate.

Scene 1: A study in Lammermoor Castle. Enrico plots to force Lucia to marry Arturo, a marriage Enrico has already committed her to because it will bolster the family’s fortunes, and to force her simultaneously to admit that she loves Edgardo. Enrico has Lucia come to him and shows her a forged letter which leads her to believe that Edgardo has betrayed her. Then he insists she marry Arturo. Lucia’s anguish is almost hysterical.

Scene 2: The great hall of Lammermoor Castle. The marriage is anticipated and Arturo is welcomed by the jubilant people. but Lucia enters for the ceremony in a very different mood, barely able to control herself, barely able to go through with it. Arturo is disturbed. Enrico, almost hysterical with anxiety himself, forces the ceremony to conclusion only instants before Edgardo bursts in to try to stop it. Edgardo is only prevented from a suicidal, single-handed attack on all by the intervention of Lucia’s old tutor. Lucia, suddenly realizing that she was not betrayed by her lover but instead was duped into betraying him, loses her mind.

Scene 1: The great hall of Lammermoor Castle. The people have resumed celebration of the wedding after Enrico has packed bride and groom off to their bedroom and departed himself in pursuit of Edgardo when the tutor enters with the shocking news that Lucia has murdered Arturo in their bridal bed. Lucia enters insane, raving about Edgardo. After giving full expression to her grief-stricken madness, she collapses.

Scene 2: A cemetery. Edgardo keeps company with the dead while awaiting an ultimate encounter with Enrico. Lucia’s mourners find him and he learns that she died of her madness. Edgardo feels grief and guilt, begs forgiveness from Lucia’s spirit and, hoping to join it, kills himself.