Fujisako Aiko is a nice but somewhat solitary student at Enishida Academy, a beautiful boarding school isolated on a lush, walled‑off island. She has grown accustomed to being alone, always trying to be a good girl, which keeps classmates at a distance. One day while wandering through the woods she catches sight of two foreign‑looking girls around her age: one is cold and strikingly pretty, the other cheerful and happy. Aiko befriends them and soon learns they are sisters from the royal family of a distant kingdom called Spinel. The family has come to Japan and enrolled at the academy under circumstances that are initially kept from her.

As Aiko is drawn into their luxurious but complicated world, she meets the full Spinel household – the sharp‑tongued princess Ruby, her bubbly younger sister Pearl, the more reserved Beryl and Garnet, their older brothers Sapphire and Lazurite, and their older sister Emerald. Aiko finds herself helping with their daily troubles, from finding a lost Garnet to standing up to Lazurite when he is harsh with his siblings. Her involvement, however, makes her the target of increased coldness from the other students, who already kept their distance.

Aiko becomes especially preoccupied with Ruby, the beautiful but scornful and hurtful princess. Ruby rarely speaks Japanese to her family, often refuses to engage, and seems heartbreakingly alone despite her royal status. Aiko slowly realizes she has fallen for Ruby, but the feeling appears unreturned. Meanwhile, the family harbors a secret that weighs heavily on them; Aiko learns of it and struggles with how to reach Ruby’s heart.

Over the course of a semester, Aiko’s classmates inexplicably begin to warm up to her, and her own family situation remains unresolved. At an end‑of‑semester party, Ruby’s feelings are finally hinted at when, after Aiko falls asleep from exhaustion, Ruby quietly kisses her. Shortly afterward, some members of the Spinel royal family leave the island, their departure marked by fireworks and a lingering sense of unfinished business. The story follows Aiko as she navigates the quiet, ambiguous aftermath of this encounter, with the fate of her connection to Ruby left open.