Chaos;Head's story is set in 2009 in Shibuya and is centered around Takumi Nishijō, a high school student at the private Suimei Academy, and the strange and brutal murders that have recently occurred in the Shibuya area, known as the "New Generation" Madness. The story begins on September 28 with Takumi talking to an online friend called Grim. Grim is trying to bring Takumi up to speed about the recent New Generation events since Takumi is not interested in local or international news. A person named Shogun joins the chat room midway through Grim and Takumi's conversation. After Grim leaves the chat room, Shogun begins to talk. He speaks in a very cryptic and confusing manner, leaving Takumi nervous. Shogun then sends Takumi a barrage of image links, one of which appears to be of a brutal murder of a man being pinned by stakes to a wall.
The next day, Takumi comes across a real life murder scene. Witnessing a mysterious pink-haired girl pinning a man up by stakes in an alleyway, he immediately realizes he is witnessing the very murder detailed in the pictures he received the day before. Convinced that Shogun is targeting him, and that the girl he saw committing the murder is somehow connected to him, Takumi tries to avoid getting involved with other New Generation events. However, as Takumi begins attempting to distance himself from the crime he witnessed, he soon catches interest of the police.
With the police suspecting him, and himself convinced that he is being targeted by Shogun, Takumi finds himself caught in a spiral of hallucinations and paranoia, as he desperately tries to keep himself safe and figure out why he is apparently being targeted. He eventually comes into contact with several other people related to the events, including the girl he saw committing the murder herself. Unsure of what is real or who he can trust, Takumi finds himself thrust into a much larger scheme than he ever thought possible, as the mysterious company NOZOMI carefully plots something from behind the scenes. (from Chaos;Head's Wiki page)