She visits the Count, and the Count shows her the document Fernand sent the police so many years before, falsely accusing Edmond Dantès of a crime he did not commit.
Mercédès is completely undone, she pleads for the Count's forgiveness, and she clearly still loves Edmond Dantès with every fiber of her being.
She asks the Count to spare her son's life at the duel the next day.
The Count is melted by Mercédès display of love and affection, and he agrees to spare her son's life. But the duel must go on (it is a question of honor and reputation), and so the only possible solution is that he, the Count, must die the next day.