At first it seemed like a Korean Shawshank Redemption set in the 1890s. And there is much of that. I had no idea watching it that this was based on a true story until the end. But it's based on the fascinating life of Kim Koo, with a long Wikipedia article about his accomplishments. This chooses a short period of his life, his time in prison as its focus.
He's a folk hero and revolutionary for the Korean people, and the movie presents an uncompromisingly idealized picture of him, as a man of honor willing to fight for his people against Japanese encroachment.
It was utterly captivating start to finish. With transcendent themes about the power of education. It's how their captors keep people down and how they learn to rise up. And about nationalism. The struggle to maintain a national identity when faced with a larger and more powerful colonial nation.