Plot

In the late 2020s, the world is in turmoil, with the United States having been fractured as a result of prolonged conflict, and a pandemic of the "St. Mary's Virus" ravaging the European continent. The United Kingdom remains the only stable country, being ruled by the fascist Norsefire party, referred to simply as "the party". Under its regime, society is heavily oppressed. Political opponents, immigrants, Muslims, homosexuals and other "undesirables" are imprisoned in concentration camps.

Evey Hammond is the daughter of parents who became activists after her brother perished in the St. Mary's school terrorist attack fourteen years earlier; they were detained and later died in prison when she was 12 years old.

On 5 November, Guy Fawkes Night, Evey Hammond (Natalie Portman), a woman who works at the state-run British Television Network (BTN), is rescued by a Guy Fawkes-masked vigilante who identifies himself as "V" (Hugo Weaving), from an attempted sexual assault by members of the "Fingermen" secret police while she is out past curfew. He leads her to a rooftop to watch his destruction of the Old Bailey criminal court building, accompanied by fireworks and the 1812 Overture.

Inspector Finch (Stephen Rea), Scotland Yard's chief of police, is given the task of investigating V's activities, while the party uses the BTN to declare the incident an "emergency demolition". V hijacks the state-run television network BTN to address the nation, claiming credit for the attack and encouraging the populace to resist Norsefire by joining him outside the Houses of Parliament on Guy Fawkes Night in one year's time. Evey is knocked unconscious while aiding V's escape, and he takes her with him to avoid her arrest and likely execution. During the broadcast, the police attempt to capture V. Evey helps V escape, but is knocked out in the process.

V takes Evey to his home, where she is told she must remain until 5 November the following year. V kills Norsefire chief propagandist Lewis Prothero (Roger Allam), coroner Dr. Delia Surridge (Sinéad Cusack), and, with Evey's assistance, Anthony Lilliman (John Standing), the Bishop of London, pedophile and corrupt priest from Larkhill, whom V gets to by using Evey.

After learning that V is killing government officials, she escapes to the home of her boss, comedian and talk show host Gordon Deitrich (Stephen Fry). In return for Evey trusting him with her safety, Gordon reveals to her a collection of prohibited materials including subversive paintings, an antique Quran, and homoerotic photographs by Robert Mapplethorpe. Gordon explains that he acts the part of a womanizer to conceal his homosexuality, well aware that if he were honest, he would be taken away by the "Finger" (Britain's secret police) and executed.

Inspector Finch learns that V is the result of experimentation on humans, and his targets tortured him while he was detained. After Gordon performs an over-the-top satire of the government on his show, his home is raided and Evey is captured while trying to escape. She is incarcerated and tortured for days for information about V, with her only solace being a note written by another prisoner, actress Valerie Page (Natasha Wightman), who was arrested for being a lesbian.

Evey is told that she will be executed unless she reveals V's location, but when she says she would rather die she is immediately released. It is revealed that her imprisonment and torture were staged by V to free her from her fears. The note was real, having been passed by Valerie to V years earlier when he was similarly imprisoned. V had intercepted Evey before Creedy's men and subjected her to false imprisonment (and gas-lighting) so she could learn to live without fear. Although initially angry at V, Evey realizes that he has been avenging Valerie and the other Larkhill victims and promises to return to see him before 5 November.

Assigned to capture V, Chief Inspector Eric Finch uses Surridge's journal and information from former covert operative William Rookwood (V in disguise), discovering that, two decades earlier, Surridge led biological weapons research and human experimentation at the Larkhill Detention Facility on behalf of then-Undersecretary of Defense Adam Sutler (John Hurt), creating the "St Mary's Virus".

Although dozens of political prisoners died during experimentation, an amnesiac in cell "V" developed mutated immunity and disfigurements as well as physical enhancements and eventually destroyed Larkhill during his escape. Peter Creedy, head of the secret police, faked a terrorist attack by releasing the virus at targets including St. Mary's. The deaths of 80,000 people and the resulting fear enabled the Norsefire party to win the next election, silence opposition, and turn the country into a totalitarian state under Sutler's rule as High Chancellor. Simultaneously, the company manufacturing the cure enriched leading party members such as Prothero and Lilliman.

Shortly after Finch is told that Rookwood has been dead for years; It was actually V in disguise, and Finch initially disbelieves his story.

As 5 November approaches, V has hundreds of thousands of Guy Fawkes masks distributed across the nation, leading to a rise in masked, anonymous chaos and eventually riots after the secret police kill a young masked girl. V shares a dance with Evey before leading her to the shuttered London underground he restored over the previous decade. Not intending to survive the night, V bequeaths the decision to start the explosive-filled train to Evey. Although she pleads that he abandon his crusade and leave with her, he refuses.

To kill the otherwise unreachable High Chancellor, V convinces Creedy (Tim Pigott-Smith) to betray Sutler and replace him in exchange for V's surrender. But after Creedy executes Sutler, V reneges on his deal to surrender and kills Creedy and his men, effectively decapitating the Norsefire government. Surviving many shots but mortally wounded, he returns to Evey to thank her and tell her he loves her, then dies in her arms.

As Evey places V's body aboard the train, she is found by Finch. Disillusioned by the party's regime, Finch allows Evey to send the train on its way. Thousands of unarmed Londoners wearing Guy Fawkes masks march towards Parliament. Without a living command structure and with no orders coming, the military general allows the crowd to pass. Everyone witnesses the destruction, which V arranged in the same style as the Old Bailey. Finch asks Evey for V's identity, to which she replies, "He was all of us."