Kino lives with his family on a farm on the side of a mountain in Japan while his friend, Jiya, lives in the fishing village below. Though everyone in the area has heard of the Tsunami no one suspects that when the next one comes, it will wipe out Jiya's entire family and fishing village below the mountain. Jiya soon must leave his family behind in order to keep the fisherman traditions alive.
Jiya, now orphaned, struggles to overcome his sadness and is adopted into Kino's family. He and Kino live like brothers and Jiya takes on the life of a farmer. Even when the wise Old Gentleman offers Jiya a wealthy life at his rich castle, Jiya refuses. Though Jiya is able to find happiness again in his adopted family, particularly with Kino's younger sister, Setsu, Jiya wishes to live as a fisherman again as he comes of age.
When Jiya tells Kino that he wishes to marry Setsu and return to the fishing village, Kino fears that Jiya and Setsu will suffer and it is safer for them to remain on the mountain as a farmer, thinking of the potential consequences should another big wave come. However, Jiya reveals his understanding that it is in the presence of danger that one learns to be brave, and to appreciate how wonderful life can be.