The story begins with Hideo Suzuki, a 35-year-old manga artist assistant, whose life seems to be stuck around his exhausting but low-paying job, unfulfilled dreams, strange hallucinations and unsatisfying relationships. He sees himself as a supporting character in his own life, and has low self-esteem, resulting in frustration.
One day, the world as Hideo knows it is shattered by the presence of a disease that turns people into homicidal maniacs that resemble and behave like zombies, whose first instinct is to attack and devour the nearest human. Armed with only his sporting shotgun, he runs for his life, meeting strangers along the way. For a while, he and his companions struggle to stay alive, while questioning their moral choices. In the end, only three of them remain and drive all the way to the top of Mt. Fuji to be saved.
Hideo Suzuki: A down-on-his-luck manga artist struggling to publish his own work while maintaining both a day job as another artist's assistant and a strained relationship with his girlfriend. Hideo is slightly paranoid and mentally disturbed, often having conversations with an imaginary figure named Yajima, hallucinating the presence of faces around him, and fantasizing about how conversations with his co-workers and girlfriend should proceed. Though he was bitten on the hand by his girlfriend, the bite did not break the skin and draw blood, narrowly avoiding infection. He seems to have gained the mysterious ability to manipulate the movements of the infected like a puppeteer. After Hiromi leaves, Hideo travels into the city and gets involved in a four-way standoff between himself, the Kurusu Cult, a group of survivors, and the giant creature that Hiromi joined with.
Hayakari Hiromi: A teenage girl who is empathetic towards the undead, shown to apparently be able to kill them by giving them closure with their life. Later, she is infected but still retains most of her personality. Post infection, she loses all the superficial symptoms of the infected (protruding black veins, bloodshot eyes). She eventually has sex with Hideo before allowing the infection to take her and joins the infected hivemind, allowing herself to be consumed by a massive building sized creature, whereupon her more base desires take control and she aids the creature in its attempt to murder Hideo, since he refuses to willingly join the infected. At the last second before devouring him, however, Hiromi's original self fights its way back to the surface and she convinces the creature to let Hideo live, under the pretense of allowing him to suffer more.
Yabu Oda: A former nurse who was reduced to the role of a sex slave for the group staying at the Gotemba Premium Outlets. She escapes with Hideo and Hiromi, and she understands the significance of Hiromi's condition in potentially understanding and curing the disease. She and Hideo have sex, whereupon she discovers that she is pregnant with the child of one of the men from the outlet. She was subsequently infected by an infant and forced Hiromi to kill her and the child by crushing them in a nearby garbage truck. However, it is revealed after Hiromi joins the infected that she was subconsciously glad that she was allowed to kill Oda, since she was jealous over her and Hideo having sex.
Kurusu: The main human antagonist. A young adult who gained superhuman strength, regeneration, and speed following his infection by his mother, whom he beat to death in an online video to display his powers and how to kill the infected. He rallies disenfranchised youths via a message board to "create a new world". He was assumed to have been killed in a three-way fight with a fellow immune person known as the "Naked Emperor" and a member of his cult named Takeshi, who had gained powers similar to his own. He later appears to Hideo after Hideo is partially swallowed by a zombie. He reappears with his cult in Tokyo trying to overthrow a surviving group, having survived his fight with Takeshi and the Naked Emperor. Before he gained his powers, Kurusu had spent most of his life in a comatose state, unable to communicate yet fully aware of the world around him.
Kurusu's cult: Disenfranchised youths who communicate on message boards and worship Kurusu. They mostly stay at home until are recruited by Kurusu himself and survived the outbreak by virtue of being shut-ins.
Other than the major characters, two characters have been used to display the ZQN epidemic in other parts of the world as they and the people around them fall to the infection. Kazu, Hideo's colleague, experiences the outbreak in Taipei and is infected together with his girlfriend. Jan, a Belgian, went back to Brussels to find his son, where he suddenly lost his memory inexplicably. After travelling around, seeing several zombies and a mutated one, he ends up in the Notre-Dame-de-Lorette, and his fate is left unknown when the church is presumably destroyed by airstrikes as he begins to show signs of infection. Another sidestory follows an immune little girl in Italy who is trapped in a belltower with a group of zombies, only to be rescued by another immune man, who eats the flesh of the infected, and who takes it upon himself to protect and raise the little girl.
The manga is serialized in the Japanese manga magazine Big Comic Spirits published by Shogakukan starting in 2009. Shogakukan has compiled the chapters into 21 volumes so far starting from August 28, 2009. It is also published in Italy by GP Manga, in France by Kana, in Spain by Norma Editorial, in Mexico by Panini Manga, in Germany by Carlsen Manga, and in Indonesia by Level Comics. Dark Horse Comics acquired the U.S. rights, with plans for a 2-in-1 omnibus release starting in 2016.
It was nominated for the 3rd, 4th, and 5th Manga Taishō awards and won the 58th Shogakukan Manga Award in the General category. In a list of "10 Great Zombie Manga", Anime News Network's Jason Thompson placed I Am a Hero at number 1, considering it "probably the greatest zombie manga ever".
As of November 30, 2015, the series had sold 4 million copies.