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.
Kotoha Ninomiya (Manami Higa) works in the general affairs department at a confectionery company. Suddenly, she receives an order to transfer to the public relations department. She has a hard time there. Making things worse, Kotoha Ninomiya learns that Atsushi Imagawa (Dai Watanabe), whom she holds secret feelings for, is about to marry another woman. They have been friends since they were young. Heart broken, she attends Atsushi’s wedding ceremony. At the ceremony, she is moved by Kumi Kuon’s (Kyoko Hasegawa) congratulatory speech. Kotoha Ninomiya is a legendary speech writer.
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.
Shinichi Sakurai (Sadao Abe) is married to Kanako (Kyoko Fukada) and they have a daughter named Kaori (Mikuu Yamada). Both, Shinichi and Kanako received only a middle school level education. Even though they don't have much money, they are a happy family. At work though, Shinichi experiences effects of only having a middle school education.
One day, Shinichi sees Kaori’s poor score for a national test. He freaks out and decides to improve Kaori's education. Thus, 41-year-old Shinichi begins studying with Kaori and instructs her for the upcoming middle school entrance exam.
Mikuri Moriyama (Yui Aragaki) is 25 years old. She doesn't have a boyfriend and doesn't have a job even though she holds a graduate degree. Everyday, Mikuri Moriyama has a feeling that she is not needed. Her father, unable to see her suffering anymore, arranges for her to work as a housekeeper. Her employer is 36 year old Hiramasa Tsuzaki. He is single and works for a company. A series of events leads them to marry as a working condition.
Kazuya Takajo (Sho Sakurai) was a promising soccer player. At the age of 23, he was selected for the national A football team and he bought a house to live with his fiancé Miki Nakagawa (Masami Nagasawa). One day, Kazuya Takajo gets into a car accident and becomes paralyzed. He must now spend the rest of his life in a wheelchair. He falls into deep despair. At that time, Miki shows him their marriage certificate.
Kazuya goes through rehab on the strength of his wife's love. Yet, he becomes frustrated by those opposed to his marriage with Miki and others who give him pity looks. He also can't find anything to replace soccer in his life. Kazuya then witnesses a wheelchair basketball game at a gym.
Yoichi Haruno (Hiroshi Tamaki) is 35-years-old and works for a company. His wife, Chisato (Kana Kurashina), is 28-years-old. Before they had a child, their marriage life was easygoing. After having their child, their relationship changes. Yoichi feels that Chisato is cold to him, but he doesn't know why.
Botchan (Kazunari Ninomiya) has sense of strong justice and does not lie. He is offered a teaching job at a middle in a small village far away from his hometown of Tokyo. Botchan accepts the position and begins to work as a math teacher. The assistant principle there wields absolute power. Botchan has a hard time dealing with his fellow teachers and the students.
Sayaka Kudo (Kasumi Arimura) is a 2nd grade high school student. She is a gal who wears miniskirts and has her hair dyed blonde. Even though she is a high school student, her actual academic grade level is on par with 4th grade elementary school students. During the summer, Sayaka attends a private institution. There, she meets Teacher Tsubota. Besides her mother, Teacher Tsubota is the first adult to praise her. Sayaka begins to change. She studies hard and eventually passes an entrance exam for Keio University.
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.