Napping Princess intertwines two distinct worlds: a near-future Japan and the magical realm of Heartland. The protagonist, Kokone Morikawa, experiences vivid dreams of Heartland during her naps. Within these dreams, Heartland is a car-centric society where the King's castle operates as a car factory, and the populace is engaged in the relentless creation of vehicles. The King has a daughter named Ancien, who appears in her younger form in Kokone's dreams. This resourceful girl possesses a "magic tablet" that brings to life various mechanical creations, including a blue toy dog named Joy and a motorcycle called Heart. However, the King forbids Ancien from animating Heart, seizing her tablet and confining her to a tower, as their society faces a dire challenge from the Colossus, a monstrous creature of molten metal. To combat this threat, the King constructs massive robots, but Ancien, with the help of a motorcycle-riding subject Peach, hatches a plan to defeat the Colossus, all while the King's advisor Watanabe schemes against both the King and Ancien.
Meanwhile, in Japan, Kokone is diligently wrapping up her school year, contemplating her college options, and reconnecting with her childhood friend Morio, who has just finished his first year in Tokyo. Living with her single father, Momotarō, Kokone often reminisces about her deceased mother, questioning her father's reluctance to share more about her. Unbeknownst to Kokone, Momotarō, a quirky car mechanic with parallels to the characters in her dreams, is implicated in a scandal just three days before the 2020 Tokyo Olympics, accused of stealing secrets from Shijima Motors. A mysterious figure resembling the King's advisor appears at their home, searching for both the tablet and Kokone. After receiving a warning text from jail, her father instructs her not to trust Watanabe. The message includes a photograph featuring a young Momotarō, his wife, and Watanabe.
With Morio's assistance, Kokone eludes Watanabe's agents and recovers her stolen tablet and a teddy bear, escaping on their family's motorcycle. The duo finds a business card for Isshin Shijima, the chairman of Shijima Motors, contemplating a visit after discovering Kokone's mother, Ikumi, was his daughter. After a shared nap on the motorcycle, they drift into Heartland, waking to find themselves in Osaka having traveled overnight. Kokone attempts to use her tablet to contact her father, only to discover that the tablet's powers extend to the real world when they receive Shinkansen tickets to Tokyo as if by magic. As they journey, they uncover the complicated history surrounding Ikumi, who had once worked on self-driving car technology, much to her father's dismay.
In her dreams, Kokone finally sees herself as more than just an observer, realizing the tales of "Ancien and the Magic Tablet" recounted by her father were her mother's stories. In Heartland, a twist unfolds as Watanabe reveals his treasonous plans to undermine the King and replace him with his robot. The King orders Watanabe's arrest while Peach engages in a fierce battle with the Colossus, aided by Kokone and her tablet. Despite a setback triggered by Watanabe, which nearly leads to disaster, Kokone and Peach triumph in an explosive confrontation with the Colossus, both in Heartland and the real world.
As the credits roll, audiences are treated to glimpses of Momotarō and Ikumi's love story alongside the early trials of her groundbreaking software, encapsulating their shared legacy that echoes through time. In a climactic moment, Kokone understands her name reflects the company's motto—a beautiful fusion of "heart" (kokoro) and "wing" (hane)—and realizes that her mother’s spirit guides her, ensuring a triumphant conclusion for the Morikawa family.
