The background and identity of V is largely unknown. He is at one point an inmate at the infamous "Larkhill Detention Centre", one of, if not the worst of many concentration camps where political prisoners, homosexuals, Jews, Pakistanis, Muslims and probably all non-Europeans are exterminated by Britain's new fascist regime, Norsefire. While there, he is part of a group of prisoners who are subjected to horrific medical experimentation, conducted by Dr. Delia Surridge, involving artificially-designed hormone injection. Lewis Prothero is the camp's commandant, and a paedophile priest, Father Lilliman, is at the camp to lend "spiritual support". All prisoners so injected soon die under gruesome circumstances, with the sole exception of "the man in room five" ("V" in Roman numerals). During that time, the man had some level of communication with Valerie Page, a lesbian former actress, kept in "room four", who wrote her autobiography on toilet paper and then pushed it through a hole on the wall.
Although there is nothing physically wrong with him, Surridge theorizes that his mind had been warped by the experimentation. Still, his actions seem to maintain a twisted logic to them. The experiments actually yield some beneficial results: he develops Olympic-level reflexes, increased strength, and incredibly expanded mental capacity (as demonstrated consistently throughout the novel, V is a genius in the fields of explosives, martial arts, philosophy, literature, politics, computer hacking and chemistry).
Over time, the man is allowed to grow roses (Scarlet Carsons) and raise crops for camp officials. The man eventually starts taking surplus ammonia-based fertilizer back to his cell, arranges it in bizarre, intricate patterns on the floor. He then takes a large amount of grease solvent from the gardens. In secret, the man uses the fertilizer and solvent to make mustard gas and napalm. On a stormy night (Nov. 5th), he detonates his homemade bomb and escapes his cell. Much of the camp is set ablaze, and many of the guards who rush in to see what happened are killed by the mustard gas. The camp is evacuated and closed down. He adopts the new identity, "V", and dons a Guy Fawkes mask and costume. V then spends the next five years planning his revenge on the Norsefire government, building his secret base, which he calls the "Shadow Gallery". He then kills off most of the over 40 surviving personnel from Larkhill, making each killing look like an accident. However, he saves Prothero, Lilliman and Surridge, (the 3 most responsible for the experiments on him) for last, showing only the remorseful Surridge a bit of mercy by injecting her with a painless poison in her sleep.
V's true identity is a mystery, and he visibly removes his mask only once during the entire story (at Surridge's request), at which point his back is to the reader. Surridge tells him "it is beautiful", in contrast to her personal notes, where she mentions that the man in cell V was ugly.
He doesn't even consider "V" his "name", saying "I do not have a name. You can call me V". The only explanation given regarding V's past is Surridge's diary, which V leaves out in the open for the "Finger" (Norsefire's secret police) to find after he kills her. Inspector Finch, the head of London's police department and one of Norsefire's most powerful officials, reads through the diary, but points out that V wanted them to read it. V also tore out many pages, which possibly left clues to his true identity before arriving at the camp. Finch further speculates that V fabricated the version of Surridge's diary, which he left with her body, just to confuse the police.
In the comic and graphic novel, Delia Surridge states in the diary that he was ugly and possibly deformed, although she mentions "Physically, there doesn't seem to be anything wrong with him. No cellular anomalies, nothing". His confidante Evey Hammond speculates in the comic that V might be her own father, who was arrested years before as a political prisoner; V denies it, however, and Moore has confirmed that V is not Evey's father. There is also some speculation that V could actually be Valerie, the prisoner in the cell next to his whose autobiographical letter inspires V not to give up (and which he later passes on to Evey). However, Prothero and Surridge both describe V as the "man" from room five (V claims Valerie was the "Woman in room four" and also that he did not write Valerie's letter), which would seem to dispel this theory.
As Finch comments on the pages V tore from Surridge's diary "What was on the missing pages, eh? His name? His age? Whether he was Jewish, or homosexual, or black, or white?". He later describes himself to Finch as "an idea". That Evey genuinely takes over the persona upon the death of her predecessor suggests V is something transcending the individual physically donning the mask, akin to Doctor Mabuse, the Phantom or the Dread Pirate Roberts. Late in the story, Evey, having taken over V's mission, also appears to consider herself as anarchy incarnate. In effect, V is an Everyman: potentially, anyone oppressed by their government could become a revolutionary avenger.
Four years after his escape from Larkhill, V begins his vendetta against the government by blowing up Parliament on November 5, Guy Fawkes Day. V then kidnaps Prothero, who is now the "Voice of Fate" on the government's propaganda radio, and drives him insane by destroying his prize doll collection in a satire of the exterminations that occurred at Larkhill. V kills now-Bishop Lilliman by forcing him to eat a communion wafer laced with cyanide. Norsefire had infused a perversion of religion into their rhetoric, saying that those who were exterminated were not pure in the eyes of God, and V's black joke was in forcing Lilliman to put some of his religious rhetoric to the test of transubstantiation. V then kills Surridge, the one Larkhill official who feels remorse for her actions, by injecting her with a poison that painlessly kills her. Having thus leveled the playing field, V moves his plans forward.
V stages an attack on the government's propaganda broadcasting station, strapping himself with explosives and forcing the staff to follow his orders under threat of detonating them. V then broadcasts a message to the people, telling them to take responsibility for themselves and rise up against their government. Finally, V destroys the government's CCTV surveillance buildings, eroding its control over British citizens. However, V is mortally wounded when he is shot by Finch, and he staggers back to the Shadow Gallery, where he dies in Evey's arms. Evey then puts him in state, surrounded by lilies and gelignite, in an Underground train that stops at a blockage along the tracks right under 10 Downing Street, where the explosives-laden train detonates, giving V a viking funeral, fulfilling his final request to her in the process. Evey then takes on the mantle of "V."