Prologue/Reincarnation Arc

An obstetric doctor and an idol fan are killed and reincarnated as twins born to the idol Ai Hoshino, beginning their unexpected new lives with memories of their past.

The Prologue and Reincarnation arc establishes the extraordinary premise of Oshi no Ko while grounding it in emotional and psychological reality that makes the fantastical elements feel consequential and meaningful rather than absurd contrivance designed for narrative convenience. Gorou Amamiya, a talented obstetric doctor renowned for his dedication to his patients and his encyclopedic knowledge of childbirth procedures and medical practice, and Ai Hoshino's most devoted fan—a young woman whose entire existence is organized around loving her favorite idol and following her career—meet in Ai's hospital when Gorou attends the birth of Ai's twins as the attending obstetrician responsible for the delivery. Their initial meeting occurs in a professional medical context, establishing Gorou's expertise and dedication to his craft and the importance he places on saving lives through his medical knowledge. In a shocking act of violence that shatters the arc's initial tone of medical professionalism, Ai's ex-partner murders both Gorou and the devoted fan, killing them instantly in a brutal attack that leaves the newborn twins motherless and bereaved and creates the supernatural catalyst for the narrative to follow. In an inexplicable supernatural event that defies rational medical or scientific explanation, both their souls are mysteriously reincarnated as Aqua and Ruby Hoshino, the twin children Ai has just given birth to before her sudden violent death. Retaining complete memories of their previous lives—Gorou remembers his entire medical career, his professional accomplishments, his patients, and his understanding of the world and human physiology—while the fan remembers her lifelong obsession with Ai, her parasocial relationship with the idol, and her previous existence as a devoted follower devoted entirely to her favorite's success. This retention of memory creates the series' fundamental premise: adult consciousness trapped in infant bodies, possessing knowledge impossible for children to possess, unable to communicate that knowledge to others or take action based on that knowledge immediately. The twins must navigate their new existence as idol children while understanding their impossible situation and confronting the reality of their previous deaths and rebirth. The arc's central tension derives from the twins' cognitive awareness and advanced comprehension contrasted with their physical inability to communicate those memories to others or take action based on that knowledge while remaining as infants unable to speak or move independently. Aqua possesses adult memories and understanding of the world trapped in an infant's body, allowing him to perceive situations adult observers would miss but preventing him from acting on that knowledge immediately or investigating circumstances surrounding his mother's death and murder. Ruby, possessing her previous life's emotional devotion to Ai combined with her new status as Ai's biological daughter and legitimate family member, experiences profound identity confusion as her parasocial fan relationship transforms into genuine familial relationship with the person she devoted her entire previous existence toward loving. The contradiction between her memories as a devoted fan and her reality as Ai's daughter creates psychological complexity that will drive her character arc throughout the series and exploration of what it means to love someone when your relationship to them fundamentally transforms. Crucially, the arc introduces Aqua's discovery that their mother did not die a natural death or through accident, but was deliberately murdered by someone with motive to eliminate her from existence. The mystery surrounding who murdered Ai Hoshino and why becomes the driving force propelling the entire narrative forward and providing motivation for Aqua's future actions and systematic investigation of the entertainment industry. Aqua immediately understands that discovering the truth about his mother's death will require careful strategy, manipulation, and systematic infiltration into the entertainment industry—the system that both created his mother as an idol and ultimately destroyed her through exploitation or deliberate harm by someone within its power structures. The arc's genius lies in establishing legitimate reasons why Aqua cannot immediately act on his knowledge or pursue his mother's killer despite possessing adult understanding of murder and investigation. He's physically an infant, unable to move or communicate in conventional ways beyond basic baby sounds and expressions that others interpret through the lens of normal childhood development. The adults around him cannot comprehend that their newborn possesses adult consciousness, memories, and understanding of his situation and his mother's death. Aqua must wait years before possessing the physical capability and social access to begin investigating systematically or gathering information relevant to discovering his mother's murderer and their motivations. The entertainment industry's shadow falls across the arc from beginning to end, creating atmosphere and thematic context for everything that follows. Ai's life as an idol involved manufactured persona, constant emotional suppression to maintain her public image, and systematic exploitation by industry structures that benefit from her labor while leaving her vulnerable to harm. Her death demonstrates the dark side of entertainment—that behind the glamorous public image exists danger, violence, and industry corruption that extends far beyond what ordinary fans recognize or understand about the entertainment business. The arc also establishes the twins' contrasting personalities and approaches to their new lives and their shared trauma. Aqua is cold, calculating, and strategic—approaching his new life with the analytical mindset of his previous profession as a doctor who saved lives through careful analysis and rational decision-making. Ruby is passionate, emotional, and devoted—approaching her new life with the emotional intensity of her previous incarnation's idol fandom and devotion to her favorite person in the world. These contrasting personalities create dynamic tension and different perspectives on the entertainment industry as the narrative progresses, suggesting that Aqua and Ruby will pursue fundamentally different paths despite their shared experience and intertwined fate through reincarnation together. The Prologue arc accomplishes the extraordinary feat of establishing a science fiction premise while grounding it in emotional reality and creating narrative momentum through genuine mystery and unanswered questions about reincarnation mechanics and supernatural forces. Rather than treating reincarnation as a convenient plot device, the series explores its psychological and emotional consequences, establishing that Aqua and Ruby must process profound trauma while navigating completely new existence with adult consciousness but infant bodies. The arc establishes Aqua's fundamental character: an adult mind determined to uncover truth and pursue justice, willing to sacrifice personal relationships and emotional authenticity to achieve his objectives.