Nanaka Kirisato, a 17-year-old high school student, is known for her serious and studious demeanor, often prioritizing academics over personal relationships. Her life takes an unexpected turn when she suffers a head injury during an accident. Upon waking, Nanaka exhibits the behavior and mindset of a 6-year-old child, despite retaining her physical age. This sudden regression creates a stark contrast between her outward appearance and her childlike innocence, leading to confusion and challenges for those around her.

Nanaka’s childhood friend, Nenji Nagihara, becomes a central figure in her life as he steps in to help her navigate her new reality. Nenji, who had drifted apart from Nanaka due to her previously aloof personality, now finds himself reconnecting with her in unexpected ways. His carefree and somewhat immature nature contrasts with Nanaka’s former self, but his genuine concern for her well-being drives him to support her through this difficult time. The dynamic between Nanaka and Nenji forms the emotional core of the series, exploring themes of friendship, responsibility, and personal growth.

As Nanaka adjusts to her childlike state, she begins to rediscover the joys of life that she had previously neglected. Her interactions with classmates, including the kind-hearted Yuriko Asaka and the mischievous Arashiko Takakura, highlight the contrast between her innocent perspective and the complexities of teenage life. These relationships provide both comedic and heartfelt moments, as Nanaka’s youthful exuberance often clashes with the more serious concerns of her peers.

The series also delves into Nanaka’s strained relationship with her father, who struggles to connect with her due to his own emotional barriers. Her condition forces him to confront his shortcomings as a parent and reevaluate his priorities. This subplot adds depth to the narrative, emphasizing the importance of communication and understanding within familial bonds.

Throughout the story, Nanaka’s journey is marked by her gradual rediscovery of her true self. While her childlike behavior initially serves as a source of conflict, it also becomes a catalyst for personal transformation, not only for Nanaka but also for those around her. The series balances lighthearted moments with poignant reflections on identity, memory, and the passage of time, creating a narrative that is both engaging and emotionally resonant.