Sanshiro is the story of the titular character's time in Tokyo as a university student and the relationships he develops. Sanshiro is a naive, country boy, out of place in the ego-centric, increasingly Westernized Tokyo. There isn't a particular plot that is tying everything together: Sanshiro is a bildungsroman.
I enjoyed the subtly of Sanshiro's slow change over the course of the novel. He eventually understands the true nature other individuals in his life, Yojiro, Professor Hirota, and Mineko, when at the beginning he was truly bewildered. I think Sanshiro's experiences ring true for almost everyone and that is what makes this story so wonderful.
Sanshiro is less intense than Kokoro. However, the character development and their emotional interplay makes Sanshiro just as engrossing. Sanshiro is a self-assuming, lovable character, trying to understand his emotions and leave his rural heritage behind. Just as in Kokoro, the main character has self-doubt about the others around him.
I really liked Haruki Murakami's introduction in the edition I wrote insightful, so if you are a Murakami fan, you may want to read the intro to gain some perspective.