Set along the Shimanto River in Kochi Prefecture, Japan. "Osozaki no Himawari" depicts the lives of seven people who follow vague dreams, but live an ordinary life.
Jotaro Kodaira (Toma Ikuta) is about to turn 30-years-old. Jotaro has recently been laid off from work and around the same time his girlfriend of seven years dumps him. Jotaro then spots a website recruiting people for work in Koichi Prefecture in a regional revival cooperation.
Kahori Nikaido (Yoko Maki) is clinician sent to work in her hometown of Koichi Prefecture. On the way to there, Kahori meets Jotaro Kodaira.
30-year-old Rinko Kamata (Yuriko Yoshitaka) works as an unpopular screenwriter. She doesn't have a boyfriend, but she has two female friends Kaori and Koyuki. They meet regularly at a bar. There, they complain about their situations and go through what if scenarios. One day, Rinko Kamata decides to go for happiness at love and work.
A postman learns that he doesn't have much time left to live due to a terminal illness. A devil then appears in front of him and offers to extend his life if he picks something in the world that will disappear. The man thinks about his relationships with ex-friends, ex-lovers, relatives and colleagues who will be sincerely sad when he dies.
Naho Takamiya (Tao Tsuchiya) is a 2nd year high school student. During the spring, Naho Takamiya receives a letter sent by herself from 10 years in the future. In the letter, she liked transfer student Kakeru Naruse (Kento Yamazaki). Kakeru Naruse carried emotional scars from his mother's suicide and he died one year later. 26-years-old Naho Takamiya thinks about Kakeru Naruse often.
2nd year high school student Naho Takamiya, with the knowledge of what she feels 10 years in the future, takes action to change her future.
Moritaka Mashiro (Takeru Satoh) doesn't want to follow in the path of his uncle who worked as a manga artist, but ultimately died because of exhaustion. Moritaka Mashiro figures he will graduate from school and work at an office. Things change though when falls in love with a girl at school. The girl, who hopes to become a voice actress, tells Moritaka they can marry, but only after they both achieve their dreams. Moritaka then teams up with fellow classmate Akito Takagi (Ryunosuke Kamiki) to publish their first manga.
Jun Hashimoto (Ren Kiriyama) is a sci-fi novel writer, but he isn't popular at all. He lives off of his savings. One day, his editor, Tateyama (Masanobu Sakata), assigns him a job. He is to write a short story for a web magazine oriented towards youths. The theme of the short story is "love." Jun Hashimoto at first refuses, because he has never written a love story before, but Tateyama pushes him to write the short story. The deadline is imminent. Jun's childhood friend Keisuke Murata (Shigeaki Kato) then appears in front of him. Jun complains to Keisuke that he can't write a love story. While talking to Keisuke, Jun comes up with an idea and begins to write his love story.
Aoi Miura (Sota Fukushi) was born and grew up in Toyama Prefecture. He has an indecisive personality and does not like summer. He was the focal member of the high school swimming team, but he now works as an assistant at a small architectural office in Tokyo.
One summer day, he meets his childhood friend and first love Akari Serizawa. During their school days they spent everyday hanging out together until the fireworks night in their 3rd year of high school. That night, Akari suddenly disappeared. Now, Akari has a man named Aoi.
Yoshikawa Nao stays behind to live by herself when her parents leave to inherit her grandpa's farm in the country. On the day she moves into her new apartment, she discovers that it was rented also to Uehara Hisashi; a cute, super-cool, popular guy from her school. Not only did their housing agency unexpectedly close down, the landlord of the apartment tells them that they had to pay more for their apartment than they had expected. With no money and no home to return to, Nao and Hisashi decide to live together as flatmates.