Monsieur d'Avrigny is the family doctor of the Villeforts. When Valentine, daughter of Monsieur de Villefort, explains to the doctor that her grandfather has died of a stroke and her grandmother hallucinates, the doctor is shocked and determines that the woman has been poisoned.
It is this doctor, M. d'Avrigny, that Maximilian overhears in the garden. The doctor informs de Villefort that Mme.de Sanit-Meran has been poisoned. He urges Villefort to "make inquiries" in the household; in the meantime, they will keep the "terrible secret." Later, after the betrothal of Valentine to Franz d'Epinay is foiled by Noirtier's revelation that it was he who killed Franz's father, an attempt is made to poison Noirtier, but the servant Barrois also drinks some lemonade and dies. The doctor is in attendance and declares the fact of poisoning, bringing the magistrate to the house; d'Avrigny suspects Valentine, but Villefort begs him to wait.
Later he is called upon to attend the poisoned Valentine. Noitier, however, tells him he has conditioned her to this poison. Still, the doctor promises to wait to notify authorities. Thus, when further attempts are made, Monte Cristo can intervene and change the drink; disguised as a priest, he cares for the "dead" Valentine, whom he has given a potion much like that taken by Juliet. Later, Monte Cristo arranges for her reunion with Maximilian.