Onikakushi-hen/Truth

The following details the actual events that occurred during. Note that all deaths and disappearances are listed only as they occur in Onikakushi-hen and apply only for the individual arc, not the overall series.

Plot
Onikakushi-hen introduces Keiichi Maebara, a student who recently transferred to Hinamizawa due to his parents' business. The chapter focuses heavily on the light-hearted school days Keiichi spends with his friends and fellow club members Rena Ryuuguu, Mion Sonozaki, Rika Furude, and Satoko Houjou. However, a photographer, Jirou Tomitake, tells Keiichi of a series of gruesome murders that occurred yearly in Hinamizawa. Keiichi attempts to learn more about the incidents from his friends, but they consistently deny that the incidents ever took place which causes Keiichi to grow suspicious of them.

Keiichi attempts to put the past of Hinamizawa behind him but it comes up again when, on the day of the Watanagashi Festival, Tomitake's friend Miyo Takano tells Keiichi about the legend of the curse of Oyashiro. That night, Tomitake is drugged by Takano with H-173, an experimental drug meant to induce terminal symptoms of Hinamizawa Syndrome. Takano and the Mountain Hounds drop Tomitake's convulsing body off near the public phone booth on the outskirts of Hinamizawa. Tomitake, as a result of his aggravated Hinamizawa Syndrome, claws his throat out in the phone booth. The next day, Keiichi learns from a detective, Kuraudo Ooishi, about Tomitake's death. Keiichi begrudgingly cooperates with the detective in order to discover the truth.

Over the course of the investigation, Keiichi meets clandestinely with Ooishi several times and grows increasingly suspicious of his friends as he learns that they all have connections to victims of the yearly murders. Concurrently, Keiichi develops worse symptoms of Hinamizawa Syndrome, compounding his paranoia. One day, Keiichi skips school under the guise of illness and is visited by Mion and Rena. The pair give him a get-well gift in the form of ohagi (in the Steam translation, the ohagi is described simply as mochi). Keiichi, afflicted by Hinamizawa Syndrome, begins to hallucinate that Mion and Rena being angry at Keiichi meeting with Ooishi. After Mion and Rena leave, Keiichi eats an ohagi and hallucinates there being a pin inside one.

The following day, Keiichi arrives at school early in search of a self-defense weapon and ends up taking a metal bat from Satoshi Houjou's old locker in the classroom. Keiichi carries the bat constantly and practices swinging it as a ruse to throw off suspicion. Rena and Mion begin to worry, as Satoshi did the exact same thing a year earlier when his Hinamizawa Syndrome symptoms worsened. Keiichi's paranoia turns into insanity as he hallucinates himself increasingly under attack from several villagers, including his friends. After hallucinating Rena showing murderous intent towards him, Keiichi runs towards the dam construction site and collapses.

After waking from his unconsciousness, Keiichi finds himself in his own room, in the care of Rena. Confused, Keiichi goes downstairs in an attempt to call Ooishi but is greeted by Mion, who ushers him back upstairs. Rena and Mion talk about calling Kyousuke Irie from the Irie Clinic to help Keiichi. In his delusional state, Keiichi believes the two to be inviting Irie to Keiichi's house to kill Keiichi using a drug that induces the same circumstances that surrounded Tomitake's death. Keiichi attacks Rena and Mion and ends up beating the pair to death with Satoshi's bat. Paranoid, Keiichi spots a group of villagers outside of his house and attempts to flee. He eventually finds a telephone booth and uses it to call Ooishi. During the call, Keiichi's symptoms reach a terminal stage and he claws out his own throat, just as Tomitake did, and dies a day later in the nearby hospital.