Edmond frees the slave Haydée, daughter of Ali Pascha, and takes her to Paris. At a ball, Haydée reveals that Fernand betrayed her father. Mercedes begins to suspect that there is something wrong with the count's identity.

Edmond finds Villefort's illegitimate son Gaston, who has become a thief. Danglars reveals Fernand's betrayal in Algeria, Fernand's son Albert becomes enraged and challenges Edmond to a duel.