Boy (Taichi) is adopted by an older man (Kanda) for sentimental reasons, develops his own sentimental attachment to the river bank where he was adopted, then meets a stranger (Zama) at the same riverbank who tells him the place will be the construction site for new housing. It's a story that's nominally about family and loneliness and, to crib from Evelyn Waugh, that moment you are looking for love and open that low door in the wall that others have found before you, that moment you realize other people in lives are just forerunners for the person you will end up falling in love with.
But actually the story just is sort of disappointingly disjointed. Zama and Kanda's similarity never really goes anywhere. You never get a sense of who any of the characters are, especially not Kanda. Zama develops complexity in the last chapter only because of the plot, so who he was before the story and meeting Taichi, who knows? and Taichi develops a backbone of mysterious origin, so one can only Hand of God say that he learns and grows from his experiences.
The art is fine, lovely and wispy, and it's not like anyone really does anything too stupid or unreasonable. It's just a story that has preemptively labeled itself as poignant, but doesn't really do anything but gesture in the direction of meaning.
Byousoku Zero Mile