Haruto Ichihara, known as Haru, hails from contemporary Japan. As a child playing outdoors with friends, he accidentally kicked a tin can toward a dog, provoking an attack. Yuusuke Ninomiya intervened, shielding Haru and suffering a bite, forging an enduring bond between them. By high school, Haru thrives as his basketball team's star player, drawing admiration from female fans. His intuitive, impulsive court style clashes with Yuusuke's analytical approach, leading Yuusuke to frequently critique Haru's efficiency. Haru maintains a committed relationship with Kotona Takashina until an unknown assailant gravely injures her. During Haru and Yuusuke's attempt to rescue Kotona, a traffic accident catapults them into Ni no Kuni—a parallel medieval realm—while Kotona vanishes. Their clothing transforms upon arrival, and their phones morph into compasses.

Tasked with finding Kotona and uncovering their displacement's cause, Haru and Yuusuke journey to Evermore kingdom, encountering humans, beastfolk, and monsters. Locals dub them "Weltenwanderer" (world wanderers). Haru stubbornly rejects Ni no Kuni's reality, insisting it must be an elaborate theme park, film set, or dream. He rationalizes monsters as remote-controlled robots, clinging to skepticism despite repeated adventures.

Ni no Kuni and Haru's original world (Ichi no Kuni) intertwine, with inhabitants sharing counterparts who mirror preferences and life experiences. A vital rule governs this bond: one counterpart's death ensures the other's demise. The narrative later implies Yuusuke is Haru's counterpart, irrevocably tethering their fates across both worlds.