On their way home from basketball practice, Kotona invites Haru and Yū to a crepe cafe, but Yū, who uses a wheelchair, turns back after they encounter steps along the way. The afternoon takes a dark turn when, in a back alley, Kotona notices a black hooded figure tailing her. She calls Yū for help, but he arrives too late as the figure stabs her. Haru rushes to get Kotona to the hospital, yet both he and Yū are suddenly yanked away to an unknown world, a realm that is eventually revealed to be the kingdom of Evermore. It is there that Yū discovers an uncanny, almost miraculous ability—he can walk—something he never did in their home world.
In Evermore Castle, the boys encounter Princess Astrid, a figure whose presence mirrors Kotona’s in form and fate, bearing a curse that has left her wounded. Yū notices a shadow knife embedded in Astrid’s abdomen, and with a decisive pull he removes it, breaking the curse that shackled her. That night, Haru drinks to forget, convinced that the whole experience was nothing more than a dream, while Yū returns to Astrid’s side and is guided to the Lake of Purity, where a dance helps to banish the lingering traces of the curse’s magic. The next morning, a summons from the wary mage Gnauss and the stern King Astrum interrupts their bewildering peace. They are forced to prove themselves by engaging in swordplay against a group of assailants, a test meant to determine whether they belong to a covert faction known as the Black Banner. As the duel escalates, both boys make a reckless leap toward a blazing flame behind the king, only to be whisked away again to their own world, where they reunite with Kotona.
A few days pass, and Haru remains convinced that the entire sequence was a dream; meanwhile Yū, seeking greater understanding, searches for an old man he met in the hospital during his childhood. A nurse cryptically reveals that the man has disappeared. Kotona drops a crushing truth: she has a malignant tumor and only months left to live. Faced with an unspeakable choice, Yū suggests returning to the other world to find a cure, but Haru’s grief and anger drive him toward a darker possibility—killing Astrid to save Kotona. The tension between the two worlds and their conflicting impulses intensifies, and once again the same hooded figure appears, whisking them away to the other realm.
Haru finds himself stranded in a bleak, desolate wasteland where he encounters Galeroth, the enigmatic leader of the Black Banner who pulls him into a broader invasion of Evermore. Yū, by contrast, returns to Evermore Castle’s throne room, where Gnauss tries to discern his loyalty and assigns him a dangerous burden: prove his innocence by killing Haru. As war erupts, Astrid proposes wielding the legendary Mornstar (known as Gladion in the Japanese version) to turn the tide. Yet Gnauss and the King reveal that the Mornstar in their possession is a counterfeit; the real relic vanished long ago. The two friends clash in a tense confrontation, but just as their conflict threatens to turn lethal, a portal casts them back into their own world.
Back in their home reality, Saki—Yū’s adopted sister—is confronted by the hooded intruder at her florist shop as the boys race against time to rescue her. They escape in Saki’s car, but the man reveals his monstrous form, transforming into a gigantic spider that pursues them. Saki explains that he called her by name, and Yū suddenly pieces together the truth behind the assaults. The trio escapes, but the car is dragged into a river, and once again they are plunged back into Evermore.
In the throne room, the truth behind the escalating crisis comes into sharper focus: Gnauss is the mastermind behind the cycle of abductions, invasions, and transportations, and he is ultimately revealed to be Galeroth, a figure who once bore the name Crown Prince Sedulus Astrum. His motive is revenge against his father and the kingdom he feels betrayed him. He tries to manipulate Haru through the armor that Yū wields with his burgeoning magic, but Yū manages to halt the plan in its tracks. Astrid is seized by Galeroth, and the battle that follows pits the two worlds against each other as Haru and Astrid fight to defend Evermore. An aging mentor reappears, handing Yū a long-forgotten stick that is revealed to be the Mornstar itself. With its power, the two friends defeat Galeroth and topple the Black Banner army.
A portal opens once more, and after a tearful farewell, Haru steps through it and returns to his world, while Yū chooses to stay behind for the moment. In the aftermath, Kotona and Saki recover from the immediate danger, but their memories of Yū fade from their minds, leaving Haru as the only person who remembers him as Yū’s soulmate from the other world.
In a quiet post-credits moment, Yū, now standing beside Astrid, is crowned the new king of Evermore, signaling a hopeful, if bittersweet, ending that promises a future where memories of a shared, otherworldly bond linger beneath the surface of two intertwined lives.
