The story is told through the eyes of Dumas, as he travels to Corsica and enjoys the hospitality of a high-standing local family. There he learns that the heir of the house has a twin living in Paris, and that there exists a spiritual bond between the two brothers to such a degree that one will feel the others pain. Lucien, the brother in Corsica, has been bothered by a nagging feeling about his brother Louis, and writes a letter for Dumas to give to him upon his return to Paris. Before he departs however, Lucien shares with Dumas a family tradition that later comes into play.
Upon his return to Paris, Louis is sought out and given the letter. The two twins are chalk and cheese to some regards, Lucien a true Corsican, highly skilled with weapons and accepting to continue 400 year old vendettas where families attempt to kill off every living relative of the other. Louis is the refined educated brother, never even having fired a weapon. All is going well, until Louis is challenged to a duel by a nobleman, and he asks Dumas to be one of his seconds. The morning of the duel, Louis is calm but certain that he is to die, having been visited by his father in the night. He gives Dumas a letter to send to his brother and mother, in which he says he died of brain fever, to prevent his brother's Corsican lust for a vendetta to occur. He dies as he predicts, and the letter is duly sent. A short time after, around when the letter was due to be received by the family, Lucien appears at Dumas' residence. He felt the shot that killed his brother, and apparently Louis repented his desire for no vendetta as he appeared to Lucien and told what had occurred. Lucien then sends for his brother's vanquisher, determined and confident that he will win, due to another vision that he has had. The story then concludes at the end of the duel.
As with all Dumas stories, The Corsican Brothers is masterfully told, with great imagery and characters throughout. Although little over 100 pages, it fits in an energetic tale that starts off in the forested slopes of Corsica, and then suddenly jumps into elegant streets of Paris. It is a tale of revenge and of the sanctity of tradition. It is telling when a third of the way into the book, Lucien remarks that "Men can patch up their differences and make peace, they can take communion from the same blessed host, but dogs will never eat from the same bowl" as by the end of the tale, he is transformed into one of those dogs, out for nothing more than to avenge the death of his brother in the only way he knows how: as a Corsican.