Samurai I, subtitled Miyamoto Musashi, is the first film in the Samurai trilogy. Based on the novel by Eiji Yoshikawa, the trilogy stars Toshiro Mifune as Miyamoto Musashi, famous duellist and author of The Book of Five Rings. The film is regarded as a classic of Japanese cinema, and is sometimes called "the Gone with the Wind" of Japan".

In Samurai I, we are introduced to Takezo, a brash young orphan hoping to win fame and glory in war, and his friend Matahachi, engaged to be married. But before he can prove himself, he is betrayed by his friend Matahachi and is soon on the run from the law. Matahachi goes off and marries, leaving behind his fiancee Otsu. Takuan, a Buddhist priest, decides to capture Takezo, taking Otsu along with him. Takuan finds Takezo and convinces him to give up, then bargains with the local warlord for Takezo's life. He ties Takezo to a tree and leaves him to contemplate his life. Otsu frees him and they flee the town together. They are separated, and Takezo searches for Otsu at the local castle. He meets Takuan, a friend of the lord, and follows him into a room filled with books. Takuan locks him inside and tells him to study the books so that he can become a great samurai. Takezo is indignant at first, but soon accepts Takuan's wishes.

Three years later, Takezo is a disciplined swordsman. After taking the name Miyamoto Musashi, he decides that he must leave the castle to learn more. Meanwhile, Otsu, who has been waiting for Takezo the past three years, is saddened to learn that he does not intend to take her with him. She protests, and he relents. But when she returns, he is gone.