The wicked Fernand Mondego is Edmond Dantès's main antagonist. He is deeply jealous of Dantès for winning the heart of his cousin Mercedes, for whom he has unrequited feelings. Mondego knows that so long as Dantès is in the picture, he will never be able to be with Mercedes. So he resolves to have him destroyed. Mondego concocts an elaborate plot whereby Dantès will end up being accused of high treason against the French state.
At the request of his captain, Leclaire, Dantès delivers a letter to a wealthy man named Noirtier. Unbeknownst to Dantes, Noirtier is a supporter of the deposed emperor Napoleon Bonaparte, and his support could get him into serious trouble.
Mondego conspires with Danglars, one of Dantès's shipmates, and someone who's also jealous of him, to send a letter to Villefort, a local magistrate and son of Noirtier accusing Dantès of treason. As Villefort doesn't want news of his father's Bonapartist political sympathies to get out, he destroys the original letter and charges Dantès with treason.
There's no evidence whatsoever that Dantès is guilty of any such crime. Yet that doesn't prevent the authorities from convicting him on trumped-up charges and sending him to the hell-hole prison of Chateau d'If. With Dantes out of the way, Mondego makes his move and marries Mercedes.