Enrico visits Edgardo at his dilapidated home and taunts him with the news that Lucia and Arturo have just been married. The two men agree to meet at dawn for a duel.

Back at Lammermoor, Raimondo interrupts the wedding festivities with the news that Lucia has gone mad and killed Arturo. Lucia enters, covered in blood. Moving between tenderness, joy, and terror, she recalls her meetings with Edgardo and imagines that she is with him on their wedding night. She vows that she will never be happy in heaven without her lover and that she will see him there. When Enrico returns, he is enraged at Lucia’s behavior but soon realizes that she has lost her senses. After a confused and violent exchange with her brother, Lucia collapses.

Edgardo laments that he has to live without Lucia and awaits his duel with Enrico, which he hopes will end his own life. Guests coming from Lammermoor tell him that the dying Lucia has called his name. As he is about to rush to her, Raimondo announces that she has died. Determined to join Lucia in heaven, Edgardo stabs himself.