During the Restoration in 1816, middle-aged aristocrat Fulvio Imbriani, Marcello Mastroianni, a Jacobin who served in the Italian campaigns of the French Revolutionary Wars, is released from prison after authorities spread the rumor that he sold out the Master of his secret society of revolutionaries—the Sublime Brothers—for freedom. Promptly abducted by the Brothers, Fulvio is put on trial until they learn their missing Master had committed suicide days earlier, disheartened by the seemingly-final defeat of revolutionary ideals. The Brothers disband, and Fulvio returns in disguise to his family villa for the first time in decades. After witnessing his relatives mourn his ostensible death, he reveals himself and is welcomed back.
Soon, he's joined by his lover and fellow revolutionary Charlotte, Lea Massari, learning that she raised enough money abroad to fund an expedition to liberate the Kingdom of the Two Sicilies, currently weakened by cholera. Fulvio, though, has grown weary of their seemingly endless and unfruitful political struggle, re-evaluating the ease of aristocratic life. He unsuccessfully offers Charlotte to go to America with their child Massimiliano, who until then had been raised by peasants to keep him safe. Fulvio's sister discovers that the reunited Brothers will come to the villa to organize the expedition and reports them to the authorities. When Fulvio learns that Austrian soldiers are about to ambush his companions, he sees the opportunity to escape his commitment and does not warn them: most Brothers are killed in the ensuing skirmish, while Fulvio flees with a fatally wounded Charlotte.
The surviving Brothers — oblivious to his betrayal — track him down at her funeral, followed by the Master's young son, Allonsanfàn, Stanko Molnar. Learning that the expedition is still going, Fulvio offers to buy himself the needed guns with Charlotte's money, with which he actually plans to escape to America along with a newly reunited Massimiliano, but first he must get rid of Brothers Lionello and her partner Francesca. He goes boating with Lionello, Claudio Cassinelli, on Lake Orta, where he claims smugglers will deliver the guns: Fulvio pretends to have been scammed and unsuccessfully tries to manipulate the suicidal Lionello into killing herself to avoid facing failure, but she dies anyway when the boat capsizes during the argument. Fulvio seduces Francesca, Mimsy Farmer, to avoid being denounced to the Brothers and, after placing Massimiliano in a boarding school and using the money to pay years of rent in the case of his death, self-injures to simulate a robbery.
Fulvio and Francesca arrive in Genoa, where the expedition should set off. Here, the Brothers are moved by the story of Southern exile Vanni, who tells them how Two Sicilies soldiers buried alive his wife due to cholera, to the point of sailing for Sicily before the lack of guns can be revealed: while Fulvio is unconscious from an opium medicine for his injury, Francesca has him boarded with the others, much to his despair. After the Brothers decide to proceed with the expedition even without guns, an increasingly frantic Fulvio finds out that Vanni is infamous in Sicily for exacting revenge on many soldiers and fellow countrymen. As soon as they land, Fulvio once again betrays his fellow revolutionaries, reporting them to a priest in the nearby village of Grottole in exchange for his life. Fearing that the hungry and cholera-stricken peasants would easily join the rebellion, the priest stirs up them against the invaders, scapegoating them for the epidemic and highlighting Vanni's involvement. Easily recognizable because of their red shirts, the unsuspecting Brothers and Vanni are all lynched on the spot by the crowd.
Fulvio is leaving Grottole when he meets Allonsanfàn, the sole survivor of the massacre. Unable to accept the outcome of the expedition, he raves about a utopian brotherhood established at first sight between peasants and revolutionaries. Fulvio is dismissive, but, after hearing Grottole's bells ringing, he believes the Brothers succeeded and wears the red shirt that Allonsanfàn left behind to join them, thus being noticed and shot dead by newly arrived soldiers.
