"Tenjou no Niji" is a manga series that revolves around the life of a young girl named Niji, who lives in a world where the sky is perpetually overcast, and rainbows are a rare and mystical phenomenon. The story begins in a small, isolated village where Niji grows up under the care of her grandmother, who tells her stories of the legendary "Tenjou no Niji," a celestial rainbow said to grant wishes to those who witness it. Niji becomes fascinated by these tales and dreams of one day seeing the rainbow for herself.

As Niji grows older, she begins to notice strange occurrences in her village, including the sudden appearance of mysterious travelers and the gradual disappearance of the villagers. These events are linked to an ancient prophecy that foretells the return of the Tenjou no Niji and the upheaval it will bring to the world. Niji learns that she is the key to fulfilling this prophecy, as she possesses a unique ability to sense the presence of the rainbow.

Driven by her desire to uncover the truth and protect her village, Niji embarks on a journey across the land, accompanied by a group of companions she meets along the way. These include a skilled swordsman named Kaito, who has his own reasons for seeking the Tenjou no Niji, and a young alchemist named Sora, who is searching for a way to save his ailing sister. Together, they face numerous challenges, including hostile factions that seek to control the power of the rainbow for their own purposes.

As the group travels, they uncover fragments of the prophecy and learn that the Tenjou no Niji is not merely a wish-granting phenomenon but a force that can either bring about great prosperity or catastrophic destruction, depending on how it is used. Niji must grapple with the weight of her responsibility and the moral dilemmas that arise as she gets closer to her goal. The story explores themes of destiny, sacrifice, and the impact of individual choices on the wider world.

The narrative is punctuated by moments of intense action, emotional depth, and intricate world-building, as the characters navigate a richly detailed setting filled with ancient ruins, magical creatures, and hidden secrets. The relationships between the characters evolve over time, with alliances tested and loyalties questioned as the stakes grow higher. Ultimately, Niji's journey is one of self-discovery, as she learns to harness her abilities and confront the true nature of the Tenjou no Niji, all while striving to protect the people she cares about and the world she calls home.