Momotaro Morikawa, Kokone Morikawa's father, is a skilled auto mechanic living in Okayama. Shijima Motors agents, drawn by his expertise, accuse him of stealing proprietary technology and secure his arrest shortly before the 2020 Tokyo Olympics. The dispute revolves around a tablet in his possession, containing self-driving car software created by his late wife, Ikumi. Watanabe, a Shijima executive, pursues this tablet to claim the technology for corporate gain.

In Kokone's recurring dreams, Momotaro appears as Peach—a courageous figure in the fantastical Kingdom of Heartland. Peach wears a jacket identical to Momotaro's real-world one and rides Heart, a sentient motorcycle mirroring Momotaro's modified bike. Heartland's narrative, where Princess Ancien uses a magic tablet against threats, stems from bedtime stories Momotaro told Kokone. These tales allegorize Ikumi's struggles and innovations, though Kokone initially believes Ancien represents herself, not her mother.

Momotaro married Ikumi after she defied her father—Shijima Motors' chairman Isshin Shijima—to pursue autonomous vehicle research. After Ikumi's fatal accident, Momotaro inherited and advanced her unfinished work, embedding her legacy within the Heartland stories for Kokone. His unresolved grief fosters a taciturn nature, avoiding direct discussions about Ikumi while preserving her memory through allegory. Upon arrest, he covertly guides Kokone to protect the tablet and seek Isshin, triggering her journey to Tokyo.

During the climax, Momotaro escapes custody to rescue Kokone from Watanabe at Shijima headquarters. Their flight through the building's rafters culminates in a perilous fall paralleling Heartland's final battle, where Peach clings to Ancien (Ikumi). The autonomous motorcycle, embodying Ikumi's technological legacy, saves them, fulfilling her vow to protect her family. This act reconciles Momotaro and Isshin, mending their estrangement as they acknowledge Ikumi's contributions.