Episode 1: Cherry Blossom

Takaki Tōno quickly befriends Akari Shinohara when she transfers to his elementary school. They grow closer to each other due to similar interests and attitudes; for instance, they both prefer to stay inside during recess due to their seasonal allergies. As a result, they form a strong bond; they speak to each other using their given names without any form of honorifics, which is very unusual in Japan, even among people who are romantically involved. This fact is lost in the movie's translation to English and other languages, which reduces the implied closeness of their relationship.

Upon graduating from elementary school, Akari moves to Tochigi, due to her parents' jobs. The two keep in contact by writing letters, but despite the feelings that exist between them, they inevitably begin to drift apart. When Takaki becomes aware that his family will be moving to Kagoshima, he decides to go see Akari since they will be too far apart to visit each other at all after he moves. He also prepares a letter for Akari, revealing his true feelings. However, during the journey, he loses the letter, and a severe snowstorm continuously delays Takaki's trip by several more hours. The two finally meet, and as they share their first kiss, Takaki realizes they will never be together again. Stranded in a shed due to the snowstorm, they fall asleep after talking late into the night. Takaki departs the next morning, and they promise to continue writing to each other. As the train rolls away, Takaki regrets the loss of his letter, while Akari silently looks at a letter she had intended to give to him.
[edit] Episode 2: Cosmonaut

Takaki is now in the third year of senior high in Tanegashima, where the Tanegashima Space Center is located. Kanae Sumida, a classmate of Takaki, had fallen in love with Takaki ever since she met him in middle school, but does not have the courage to openly confess her feelings. She spends all the time she can with him, even waiting long after school for the chance to go home together. It is obvious Kanae has strong feelings for Takaki, but he appears to be blind to them; he simply regards Kanae as a good friend. Over time Kanae observes that Takaki is always writing emails to someone, or staring off into the distance as if searching for something far, far away. It is revealed later in this segment, that the emails Takaki is constantly writing are merely to himself, and he has had recurring dreams in which Akari is featured. Despite her feelings for Takaki, Kanae believes he is searching for things far greater than anything she can offer and eventually decides against telling him how she feels.
[edit] Episode 3: 5 Centimeters Per Second

It is 2008, and all three characters have gone their separate ways. Takaki is now a computer programmer in Tokyo, and Akari is preparing to get married. There is no clear indication as to what Kanae has done, but in the ending montage she can be seen slightly older laying on the ocean water next to her surfboard as well as saying goodbye to Takaki as he is leaving via airplane. Takaki is still longing for Akari to the detriment of his lifestyle and his other relationships, which a recent ex-girlfriend also acknowledges. Akari, while going through some old things, finds the letter she intended to give to Takaki but never sent. She feels nostalgic for those days but has moved on with her life. On the verge of a breakdown, Takaki quits his job.

One day while walking down a road, Akari and Takaki presumably pass and recognize each other, across a train crossing. They both pause and begin to look back, but the passing trains quickly cut off their view. Takaki waits for the trains to pass, but sees that the woman is gone. After a pause, he smiles to himself and continues walking.