Naho, an eleven-year-old girl, lives with her father and her two-year-old brother, Jun. Her mother died after Jun's birth, forcing Naho into significant caregiving responsibilities for her brother. This role chains her to household duties and Jun's needs, constantly sacrificing typical childhood activities like spending time with friends. Understandable frustration builds within Naho, who craves greater freedom and social connection.

On her birthday, her plans to meet friends shatter when her father departs for the day, instructing her to wait for a babysitter called "Helper-san." The arrival is Kotaro, an enigmatic figure identifying himself as a "Dream Sphere Dealer" or "Dreamball Dealer." He presents Naho with an offer: experience one of three distinct dreams, each promising to fundamentally alter her life.

Following this encounter, strange phenomena fracture Naho's everyday reality. She experiences sudden, jarring transitions into vivid dreamscapes. One moment she enjoys time with friends; the next, the setting shifts violently. Different worlds materialize and dissolve before her eyes, including one where she reunites with both her living father and deceased mother, momentarily restoring their original family unit. This vision brings initial comfort, swiftly replaced by rising anxiety.

Later, Naho unexpectedly plunges into Jun's dream. This intrusion into her brother's subconscious deepens her confusion about these experiences and her own desires. Confronted by Kotaro's offer and the disorienting journey through multiple realities, Naho struggles to grasp her deepest wishes amidst the conflict between her longing for personal freedom and her ingrained sense of responsibility towards Jun.