Sanjūrō Tsurugi, an 80-year-old kendo master residing in Tokyo, outwardly embodies an elderly man frustrated by his physical limitations. He acts as the strict teacher and grandfather to his 17-year-old granddaughter, Michiko Tsurugi, a high school student. His immense pride in his kendo skills surfaces when he mocks Michiko during practice, boasting that no one has ever defeated "Sanjuro the demon." However, after Michiko outmaneuvers him by tiring him out running around a magical cherry tree in his garden, deep envy flares within him towards the tree's enduring strength and beauty despite sharing his age. This compels him to fervently wish under the tree for a return to youth.
The magical cherry tree grants his wish, biologically transforming him into a teenager roughly Michiko's age (16-17 years old). Adopting the alias "Sanjuro Sakura" (referencing "cherry blossom") as a nod to the transformative tree, he revels in his revitalized body, testing his boundless energy by running tirelessly around the schoolyard. His core personality remains intact – impulsive and hotheaded – evident when he bursts into Michiko's classroom demanding a rematch without regard for the disruption. Experiencing youth anew, he explores Tokyo with childlike enthusiasm, engaging in activities like playing baseball, bowling, and eating at McDonald's, experiences missed during his original youth.
An anime-exclusive subplot sees him journey to Beika Ward. There, encountering Ran Mouri and Sonoko Suzuki in a magazine store, memories resurface upon reading a poem in Mokichi Saito's "Touson." This triggers a search for Harue, his childhood love from over 60 years prior. Guided by cherry blossoms, he locates her home and silently observes her, now an elderly woman surrounded by family. Recognition strikes upon seeing her distinctive golden flower comb, leading to a poignant moment of silent acknowledgment before he departs without interaction, accepting the past.
His protective instincts for Michiko ignite a major conflict. Overhearing student Tatsuya Nakadai and associates plotting to harass her, he intervenes and defeats Nakadai's henchmen. Nakadai, a skilled kendo fighter, challenges him to a formal duel. During their schoolyard fight, Michiko's inadvertent support for Nakadai distracts Sanjuro, resulting in a significant injury. Later, Nakadai ambushes them at home wielding a real sword, injuring Sanjuro's shoulder. When Nakadai escalates by throwing knives, Michiko retrieves her grandfather's katana and throws it to Sanjuro. He employs his signature technique, "Hikken - Cherry Blossom Storm," destroying Nakadai's uniform and severing his hair, defeating him without lethal harm, demonstrating his kendo mastery and prioritizing Michiko's safety over pride or victory.
Reflecting near the cherry tree afterward, he acknowledges the tree granted his wish but decides his time as a youth is complete, expressing contentment with the experiences gained. He subsequently reverts to his elderly form. When Michiko inquires about "Sanjuro Sakura," he teases her about potential romantic feelings, concealing his involvement. Her reply that Sakura was "much cooler" than him prompts laughter from Sanjuro, underscoring his acceptance of his age and the adventure's temporary nature.