Preparations have been made for the imminent wedding of Lucia to Arturo. Enrico worries about whether Lucia will really submit to the wedding. He shows his sister a forged letter seemingly proving that Edgardo has forgotten her and taken a new lover. Enrico leaves Lucia to further persuasion, this time by Raimondo, Lucia's chaplain and tutor, that she should renounce her vow to Edgardo, for the good of the family, and marry Arturo.
Scene 2: A hall in the castle
Arturo arrives for the marriage. Lucia seems distressed, but Enrico explains that this is due to the death of her mother. Arturo signs the marriage contract, followed reluctantly by Lucia. At that point Edgardo suddenly appears in the hall. His appearance leads to the celebrated sextet 'Chi mi frena in tal momento' in which al the conflicting emotions of the moment are graphically portrayed. Raimondo prevents a fight, and he shows Edgardo Lucia's signature on the marriage contract. Edgardo curses her, demanding that they return their rings to each other. He tramples his ring on the ground, before being forced out of the castle.