In the days when Copernicus's heliocentrism was unknown to many, Santetsu Yasui Junichi Okada is a steadfast, honest man from the Yasui family, a lineage famed for teaching Go to the shogun. He is deeply passionate about observing the stars and solving arithmetic puzzles, often to the point of exuberance as his mind catalogs patterns and possibilities. He dreams of aligning the heavens with a better measure of time, a goal that will test his patience and precision.
One day he visits Konno-Hachimanngu Shrine to study a new sangaku, where he meets En Murase Aoi Miyazaki, a woman who will later become his wife after he completes his calendar challenge. En shares with him a story about the person who answered the sangaku instantly, hinting at the kind of sharp insight that excites Santetsu and motivates his own meticulous work. Their meeting sparks a bond that blends intellect and affection as the calendar project begins to take shape.
On the same day, Santetsu’s Go rival Hon'inbō Dōsaku clashes with him in a real game—no longer a staged exhibition before the Shogun Tokugawa Ietsuna and their Go masters. The duel is interrupted by a solar eclipse, a dramatic omen that many interpret as a sign of trouble ahead. Although their masters scold them for not performing as expected, the enigmatic Ietsuna finds the improvised contest intriguing, hinting at a deeper respect for genuine skill and strategic insight.
In the broader historical frame, Japan relies on the Senmyō calendar, a Chinese-derived system that gradually slips out of sync. Hoshina Masayuki tasks Santetsu with producing a correct calendar through careful astronomical observations, a feat that must be coordinated with the Imperial Court. This bureaucratic and scientific challenge makes Santetsu’s mission especially hard and burdensome, yet it also places him at the heart of a pivotal reform.
The film follows Santetsu Yasui's life as a mathematician and inventor of the Jōkyō calendar, charting how his curiosity, rigor, and perseverance drive a important intellectual breakthrough. It is an adaptation of Tow Ubukata's novel Tenchi meisatsu, bringing to life a period when arithmetic, astronomy, and governance intersect in a quest for celestial accuracy.
Alongside the central arc, the story is enriched by a cast of historical figures: Mitsukuni Mito Kiichi Nakai and Dennai Tatsube Takashi Sasano appear as important contemporaries, while Tenchi Meisatsu Shota Sometani and Gieki Murase Ryuta Sato contribute their own perspectives to the era’s scientific and cultural milieu. Together, these characters illuminate the meticulous work of calendar reform, the tension between art and science, and the quiet persistence of a man who aimed to measure the heavens with honesty and care.
