The film is notorious amongst moviegoers, has raised widespread controversy, and is banned outright in several countries due to its high impact violence and graphic depictions of cruelty.
In a flashback, Ichi masturbates on an apartment balcony while watching a pimp assault a prostitute within.
Off-camera, sadistic crime underboss Anjo is brutally murdered. An expert cleaning crew run by Jijii (Shinya Tsukamoto) immediately removes any trace of Anjo's remains, and credits Ichi for the kill. Later, Kakihara (Tadanobu Asano), Anjo's sadomasochistic enforcer, and other crime lords visit the spotless apartment, concluding that Anjo fled town with the prostitute and ¥3 million of the gang's money.
Kakihara visits an underworld night club with other gang members. He tells Anjo's girlfriend, an English-speaking Chinese prostitute named Karen (Alien Sun) that Anjo must still be alive, though perhaps kidnapped by a rival gang. He also greets Jijii and the cleaning crew at another table, unaware they were in town, and asking them to contact him if they learn anything of Anjo's status.
Jijii feeds Kakihara rumors suggesting that Suzuki (Susumu Terajima), a member of the rival Funaki clan, has kidnapped Anjo. Kakihara captures Suzuki and tortures him with suspension and piercing, but when Suzuki turns out to be innocent, Kakihara slices off the end of his own tongue and offers it to Suzuki's boss (Jun Kunimura) as penance.
Kakihara and gang members raid a hotel room and capture Kano, a drug-addled member of the cleaning crew who has had facial plastic surgery since Kakihara last saw him, but admits his identity and past acquaintance with Kakihara when he believes he will be killed. He reveals that although he helped clean up the murder scene, it was Ichi who killed Anjo, and Kakihara has now been targeted.
Returning to the opening flashback, Ichi is stepping in from the balcony to kill the pimp brutalizing Sailor, a prostitute whom Ichi patronizes. Afterwards, he tells her that he will be the one beating her up now. When Sailor assaults him, Ichi reflexively kills her as well.
At Suzuki's prompting, Kakihara is kicked out of the syndicate, but the entire Anjo gang defects with him. Suzuki then promises Jijii a million yen to 'squash' Kakihara. Jijii, it is revealed, is secretly orchestrating events in order to pit yakuza clans against one another, with the help of Ichi. Though a normally unassuming and cowardly young man, Ichi becomes homicidal and sexually aroused when enraged. Jijii is able to manipulate Ichi's weak personality by implanting several false memories — a high school rape in particular — and uses the unstable Ichi as an assassin.
While bicycling in the evening, Ichi assaults one of three boys bullying a fourth named Takeshi. Takeshi is the son of Kaneko, one of Kakihara's henchmen. Jijii incites Ichi to enter an apartment containing several criminals of the old Anjo gang, and slaughter them all. Afterward Ichi sees Takeshi, who thanks him for the earlier protection. Kaneko finds a brothel-keeper assaulting Ichi in an alley and, remembering his own long-ago rescue by a member of the Anjo gang, helps Ichi out.
Kakihara enlists the help of corrupt twin police detectives, Jirô and Saburô (Suzuki Matsuo), to find Myu-Myu, a prostitute connected with Ryu Long, a member of Jijii's gang. When Jirô fails to get information from her through torture, Saburô sniffs her body to get Long's scent and tracks him down. Though Long outruns the brothers, Kakihara captures him and with the twins tortures him for leads to Jijii.
To turn Ichi into a complete killer, Jijii has Karen, Anjo's woman and Jijii's friend, seduce Ichi by pretending to be the woman in his false memory. When Ichi becomes confused by Karen's claims that she desired for him to rape her, he kills her. Jijii calls Kakihara to let him know Ichi is coming to kill him but is spotted by one of Kakihara's men, whom he then maims and kills.
Kaneko, Kakihara and Ichi chase each other to a rooftop. Due to Jijii's psychological manipulation, Ichi believes that Kaneko is his brother and confronts him. Kaneko shoots the side of Ichi's leg, causing Ichi to slit Kaneko's throat in front of Takeshi. Takeshi attacks Ichi as he lies on the roof begging for forgiveness. Kakihara realizes Ichi cannot hurt him and inserts skewers into his ears to drown out Ichi's cries. Suddenly he sees that Ichi has chopped off Takeshi's head. Ichi charges Kakihara, embedding one of his razor-bladed boots in the center of his head. Kakihara falls from the roof to his death. However, when Jijii finds him, Kakihara has no wound in his head; he hallucinated Ichi's attack as he jumped to his death while Ichi cried.
Years later, Jijii's corpse hangs from a tree in a park. A young man resembling an older Takeshi leaves the park with a group of schoolchildren.
Director Takashi Miike intended for the author of the original manga, Hideo Yamamoto, to write a script entirely in manga form, but the idea fell through when Yamamoto felt he could not complete it due to writer's block.
The soundtrack was written and produced by Karera Musication, a side project of the Japanese band Boredoms, under the direction of ex-guitarist Seiichi Yamamoto and percussionist/band leader Yoshimi P-We.
As a publicity gimmick, vomit bags were handed out at the Toronto International Film Festival (TIFF) to those attending the midnight screening of this film. Similar bags were handed out during the Stockholm International Film Festival. Reportedly, watching the movie caused one person to throw up and another to faint.
In January 2009, The Norwegian Media Authority learned of this incident. Based on the incident and on the assessment that its infliction of violence "is potentially harmful to children and adults" the film was banned in Norway. Any person caught screening or selling the film in Norway can face arrest with possible fines or imprisonment.
The film has been banned in Malaysia since the movie's distribution date. It later caused controversy in Germany and the film was banned for distribution there too. Private possession of the film remains legal. The film remains banned in all three countries as of 2009.
Tom Mes has suggested that the film is in fact a very sophisticated assessment of violence and its relation to the media and implicating the audience. He writes that "It's a paradox, but Ichi the Killer, a film that sets new boundaries in the portrayal of violence and bloodshed, takes a strongly critical stance towards the portrayal and the consumption of the violent image. However, it does so without ever taking a moral stance towards either the portrayal or the consumption, thus circumventing any accusations of hypocrisy on the part of the director. Miike does not moralise or chastise, but provokes the audience into questioning their own attitudes towards viewing images of violence. He steers them into a direction but leaves it up to them to draw their own conclusion".
Mes is also very critical of the edits made to the film. He argues that "The film as a whole is a completely cohesive unity, in that all of its parts are absolutely crucial to the functioning of the whole. Any attempt at censorship or toning down the violence will have the opposite effect and will in fact make the film more exploitative and thereby undermine its critical stance. Excising scenes of violence, particularly the 'painful' scenes, will harm the symbiosis between the 'playful' and the 'painful' violence, which forms the basis for Miike's critical approach".
The film is followed by a prequel, 1-Ichi. Nao Omori reprises his role in the prequel, playing a younger version of Ichi.