The narrative of No Longer Human is meticulously structured around a series of notebooks left behind by the central figure, Ōba Yōzō, portrayed by Shun Oguri. These three notebooks serve as a raw, chronological account of Yōzō's existence, spanning from the innocence of early childhood to the disillusionment of his late twenties. The story is framed by a nameless narrator who receives these documents from a mutual acquaintance a full decade after they were originally penned, providing a haunting epilogue and preface to the tragedy.
The first notebook delves into the formative years of Yōzō, revealing a child plagued by an extreme and debilitating difficulty in understanding the fundamental nature of other human beings. Despite being born into a family that is respectable and financially secure, Yōzō suffers through abuse and corruption at the hands of the household servants. Trapped in a cycle of silence, he becomes accustomed to suppressing his true emotions and doubting the intentions of those around him, which prevents him from seeking help or confiding in anyone regarding the trauma he endures.
This deep-seated sense of alienation evolves into a profound fear of people. To survive in a world that feels incomprehensible, Yōzō resorts to mimicry. He constructs a "comic character," a buffoonish persona designed to mask his internal void and secure a semblance of love and acceptance from others. He views this acting as a last resort for interpersonal connection. He frequently notes that these antics are a defensive mechanism, used specifically to avoid provoking anger or being held accountable for his perceived failures. However, this facade comes with a heavy price: an obsessive, paralyzing fear that someone will eventually see through the mask and expose his true, hollow self.
This tension peaks during his school years when a classmate, Takeichi, manages to see through one of his false displays of humor. Terrified of exposure, Yōzō strategically befriends Takeichi to ensure his secret remains safe. During this period, Yōzō discovers a kinship with the works of Van Gogh. Inspired by the artist, he begins to paint as a means of venting his inner agony. He creates a self-portrait so harrowing and dreadful that he refuses to show it to anyone except Takeichi, who ironically predicts that Yōzō is destined to be a great artist.
Upon completing high school, Yōzō moves to Tokyo to pursue university studies. It is here that he meets a fellow artist named Horiki, whose influence spirals Yōzō further into a lifestyle of excess. He descends into a destructive pattern of heavy drinking, chain-smoking, and promiscuity, while drifting into communist meetings without any real political conviction. The volatility of his life reaches a breaking point when, after spending the night with a married woman, he attempts shinjū (a double suicide) by drowning. The attempt ends in tragedy; the woman perishes, while Yōzō survives, left to carry an excruciating and permanent weight of guilt.
Following his expulsion from university, Yōzō is placed under the guardianship of a family friend. He attempts to stabilize his life by entering a relationship with a single mother, an acquaintance of Horiki. For a brief moment, he tries to embrace a normal existence, acting as a surrogate father to her young daughter. However, the pull of his old demons is too strong; his fear of humanity and his addiction to alcohol resurface, leading him to abandon the family. He eventually finds himself living with the madam of a bar, Mitsu Dan, before meeting Yoshiko, played by Erika Sawajiri. Yoshiko is a young, innocent woman who hopes to save him from his alcoholism.
For a time, Yoshiko's grounding presence works. Yōzō manages to quit drinking and finds professional success as a cartoonist. However, the reappearance of Horiki triggers a relapse into self-destructive behavior. In a cruel twist of fate, while Yōzō is discussing the philosophical antonym of "crime" with Horiki—recalling Dostoevsky's Crime and Punishment—Yoshiko is sexually assaulted by a casual acquaintance. This act of violence shatters the fragile peace Yōzō had built. The resulting terror and despair alienate him from his wife and drive him to another suicide attempt using soporific drugs.
After being rescued and released from the hospital, Yōzō falls into a deep addiction to morphine. His downward spiral leads him to be confined within a mental institution. Upon his eventual release, with the assistance of his brother, he moves to a remote, isolated location. The chronicle ends on a chillingly neutral note, as Yōzō reflects on his state of being, concluding that he now feels neither happy nor unhappy.
