"Bright Light Sprout" is a manga series that centers on the life of Haruto, a reserved high school student who struggles with feelings of isolation and self-doubt. The narrative begins with Haruto encountering a mysterious glowing plant in an abandoned lot near his home. This plant, which he names "Sprout," possesses the ability to communicate telepathically and exhibits unique properties, such as emitting a soothing light that alleviates emotional pain. As Haruto begins to care for Sprout, he discovers that the plant is connected to a larger, enigmatic phenomenon involving otherworldly beings known as "Luminaries."

The Luminaries are ethereal entities that exist in a parallel dimension, and their presence begins to intersect with Haruto's world as Sprout grows stronger. Haruto learns that Sprout is a bridge between the human world and the Luminaries' realm, and its growth is tied to the emotional energy of those around it. As Haruto forms bonds with classmates who also struggle with their own insecurities—such as Aoi, a talented artist plagued by self-criticism, and Riku, a former athlete grappling with a career-ending injury—Sprout's light intensifies, revealing hidden truths about the Luminaries and their connection to human emotions.

The plot unfolds as Haruto and his friends navigate the challenges of adolescence while uncovering the purpose of the Luminaries. They learn that these beings are drawn to human emotions, particularly those of despair and hope, and that Sprout serves as a catalyst for balancing these energies. As the group delves deeper into the mystery, they encounter antagonistic forces, including rogue Luminaries who seek to exploit human emotions for their own gain. The story explores themes of emotional resilience, the power of human connection, and the interplay between light and darkness, both literally and metaphorically.

Haruto's journey is marked by his evolving relationship with Sprout and his growing understanding of his own emotional landscape. The narrative balances slice-of-life moments with supernatural elements, creating a story that is both grounded and fantastical. The characters' personal struggles are interwoven with the overarching plot, emphasizing the importance of empathy and self-acceptance. As the series progresses, Haruto and his friends must confront their fears and work together to protect both their world and the realm of the Luminaries, culminating in a resolution that underscores the transformative power of hope and unity.