In a bitter feud between the Scottish families of Ravenswood and Lammermoor, Lord Henry Ashton has gained upper hand over Edgar of Ravenswood, exterminating his kinsmen, and taking over his estates. At the time of the opera’s action, Henry’s fortunes have begun to wane. In political disfavor, he stakes his salvation on uniting his family with that of Lord Arthur Bucklaw, whom he wishes his sister, Lucy, to marry.

In a woodland park near Ravenswood Castle, Henry’s retainers search out a mysterious trespasser. Henry learns from Norman, the captain of his guard, that Lucy and Edgar have been meeting secretly in the park of Lammermoor. Enraged to discover that the mysterious trespasser is his hereditary enemy, Henry swears revenge.

At a fountain near her mother’s tomb, Lucy awaits a rendezvous with Edgar. She tells her servant, Alice, a tale about a ghost which haunts the fountain, and which has warned her of a tragic end to her love for Edgar. Edgar arrives and explains that he must journey to France on a political mission. Edgar wishes to end the feud between the two families so that he and Lucy may marry. Lucy, knowing that her brother would never relent, begs Edgar to keep their love a secret. He yields to her entreaties, receiving her pledge of faith as the two lovers say good-bye.

Plotting with Norman to force Lucy into the marriage he has already arranged for her, Henry summons his sister and shows her a forged letter which leads her to believe that she has been betrayed by Edgar. Henry insists on her marrying Lord Arthur at once to save the family fortunes.

In the great hall of the castle, guests celebrate the union of two important families. Henry explains Lucy’s distraction as continued grief over her mother’s death. Desperate, yet reluctant, Lucy signs the marriage contract. Edgar, returning from France sooner than expected, has heard of the wedding, and bursts in to claim the bride. Seeing Edgar outnumbered by his enemies, Bide-the-Bent restrains him from a suicidal attack.

While Henry travels to Edgar’s castle to challenge him to a duel, the wedding feast continues. Bide-the-Bent enters to announce that Lucy has lost her mind and has killed her bridegroom. Her mind wandering, she appears in a disheveled state, imagining herself first married to Edgar, then parted from him. As the guests watch in hushed, pitying horror, she collapses.

Edgar waits for Henry among the tombs of his Ravenswood ancestors. Guests leaving Lammermoor Castle come upon the scene and tell Edgar of Lucy’s tragic fate. Begging forgiveness from the spirit of Lucy, whom he now seeks to join in death, Edgar stabs himself before the onlookers can stop him.