In Corsica, the Franchi clan gathers around Count Victor Franchi [Henry Wilcoxon] and Countess Franchi [Gloria Holden] as they await the birth of their child. Dr. Enrico Paoli [H.B. Warner] delivers astonishing news: the infant is Siamese twins. Count Franchi insists on a surgical separation, even though Paoli warns that it would be a miracle if the babies survive. Before surgery can begin, the bitter rivals, the Colonnas, led by Baron Colonna [Akim Tamiroff], launch a surprise attack. All of the Franchis are killed except the twins, who are carried to safety by Paoli and Lorenzo [J. Carrol Naish].
Later, Paoli successfully separates them. To protect them, Count Franchi's friends, Monsieur Dupre [Walter Kingsford] and Madame Dupre [Nana Bryant], take Mario Franchi [Douglas Fairbanks Jr.] to Paris to raise as their own son, while Lorenzo hides in the Corsican hills with Lucien Franchi [Douglas Fairbanks Jr.].
Twenty years pass. Lucien, now a bandit leader, experiences a strange bond with his brother and often feels what Mario feels, though he has not learned about the twins’ connection. At a Paris theatre, Mario saves Countess Isabelle Gravini [Ruth Warrick] from being harassed by a marquis [Henry Brandon]. When the Marquis stabs Mario in the back, Lucien likewise feels the pain.
When the twins are twenty-one, Paoli reunites them, and they vow to avenge their parents by killing Colonna, who now rules Corsica with tyranny. Lucien confirms that the bond he experiences with Mario is real. Isabelle’s world becomes entangled with Colonna’s plans, and when her father, Count Gravini [Pedro de Cordoba], refuses to sanction the match, he is poisoned. Mario warns Isabelle of Colonna’s intentions, and she flees with Mario to the bandit camp. Lucien, torn between his love for Isabelle and his loyalty to Mario, seeks guidance from Paoli but finds none, prompting him to decide he must kill Mario to live his own life. After Lucien confesses his love to Isabelle and embraces her against her will, Mario confronts him. Lorenzo breaks up the knife fight, and Isabelle, unaware of this confrontation, decides to return to Paris to avoid splitting the brothers.
Colonna’s trusted adviser, Tomasso [John Emery], finally deduces that the Franchi twins are behind the vendetta. They set a trap, using Isabelle as bait. When Lucien refuses to risk his men, Mario, posing as a French jeweler, attempts to rescue Isabelle. Maria [Veda Ann Borg], Colonna’s mistress, helps him, but he is captured and whipped to reveal Lucien’s hiding place. Colonna forces Paoli to attend. When Mario faints, Paoli administers a drug that makes it appear as if Mario has died, though he later revives.
Lucien, now believing Mario is dead, leads his band to confront Colonna. He is fatally shot in the back by Tomasso, but manages to kill Tomasso before collapsing. Then Mario appears, and in a final sword duel, the last Franchi slays the last Colonna. Before Lucien dies, he reconciles with his brother.
