Tomoya Okazaki is a third year high school student resentful of his life. His mother passed away from a car accident when he was younger, causing his father to resort to alcohol and gambling. This results in fights between the two until Tomoya’s shoulder is injured in a fight. Since then, Tomoya has had distant relationships with his father, causing him to become a delinquent over time. While on a walk to school, he meets a strange girl named Nagisa Furukawa who is a year older, but is repeating due to illness. Due to this, she is often alone as most of her friends have moved on. The two begin hanging out and slowly, as time goes by, Tomoya finds his life shifting in a new direction.
Tomoya Okazaki (Kenji Nojima) is a male third-year high school student who does not feel at home due to a distant father who works a night shift, and a mother who died when he was a child. Due to this, he simply goes to school for no reason and does not have any interest in school activities. Tomoya meets a third-year girl from his school one day early in the school year named Nagisa Furukawa (Mai Nakahara). Nagisa is repeating her last year of high school due to having been sick most of the previous year, and this year she does not know what to do. Tomoya suggests she finds something new to do at school, and she comes up with restarting the drama club. By the time the sakura trees are done blooming, Nagisa has already started hand-making posters advertising the drama club’s reformation. Tomoya, and his good friend Youhei Sunohara (Daisuke Sakaguchi), help Nagisa with putting them up around the school, but the student council quickly goes around and uses red paint to deface the posters and write ‘invalid’ on them. This angers Youhei greatly and he beats up one of the student council members.
Kouko Ibuki (Yūko Minaguchi), the teacher who had been the adviser for the drama club before it disbanded, gets involved by telling Nagisa that if she can gather more members that she will talk to the school about reforming the club. Tomoya and Youhei try again by this time making copies of hundreds of posters and putting them all around the school, but no one joins the club. In the end, Nagisa gets Tomoya and Youhei to sign up, and she reassures them that she will not ask them to act on stage. The drama club is reformed, and with a month left until the school festival, Nagisa decides to do a soliloquy based on a dream she has had ever since she was a child; meanwhile, Tomoya and Youhei work backstage with the music and lights respectively. Nagisa gets to writing the script, and invites Tomoya and Youhei over to her house for dinner, though only Tomoya can come due to Youhei taking up a part time job with an electrical company. At Nagisa’s house, which turns out to double as a bakery, Tomoya meets her energetic parents Sanae (Kikuko Inoue) and Akio (Ryōtarō Okiayu), and even gets to stay overnight after being heavily persuaded by Nagisa’s father.
Like Nagisa, Tomoya has also had a recurring dream ever since he was a child. In his dream, he initially is disembodied in a dead world where he is the only thing “alive”. He finds a discarded human-sized doll and uses it as a body to travel around the world on an old bicycle. After some searching, he finds a large sakura tree known as the “promised tree” where he believes he will meet the person he has felt is with him in this dead world.
When the school festival finally begins, Nagisa reveals that she has not finished the script, but still wants to go on with the play since the story is still within her. Nagisa has her performance in the evening, so in the meantime Nagisa hangs out with Tomoya and Youhei. During lunch, she tells them that the reason why she wanted to do drama was due to her parents formally having been theater actors, but they both had to give up acting after Nagisa was born. Nagisa wants to do drama so as to continue her parents’ dreams in her footsteps. For the play, Sanae gives Nagisa her wedding dress to use as her costume, much to the surprise of Akio. Nagisa starts with her monologue without a hitch, and during her recitation Kouko gives stage directions to Tomoya and Youhei via headsets. Gradually, Tomoya comes to realize that the story Nagisa is reciting is the same dream that he has had since he was a child, and is shocked to find that Nagisa also had the same dream of the dead world. At the play’s conclusion, Tomoya realizes that he and Nagisa were meant to be together and confesses his love to her.
Shortly after the festival, Nagisa’s health falters once again and she has to take another leave of absence from school for a year, meaning it is not until a year after Tomoya’s class graduated that Nagisa is able to graduate high school as well. After this, Tomoya and Nagisa start living together in a small apartment in town while Tomoya works for an electrical company and Nagisa works part time as a waitress at a local family restaurant. Later, Nagisa becomes pregnant, and her doctor informs her family that if she gives birth to the child that she may die in the process. However, Nagisa’s family chooses not to tell Nagisa about it. One winter night, Nagisa gives birth to a baby girl she names Ushio and Nagisa dies that same night. This causes Tomoya to go into deep depression for a period of five years, during which he does not go to work, or even visit his daughter who is being raised by Nagisa’s parents.
Tomoya’s friends and his father try to get him out of his depression, but Tomoya is very stubborn. Tomoya’s father (Hiroshi Naka) asks Tomoya’s friends to take him out on a retreat for a few days in order to break his depression, and they take Tomoya out of his house so fast he does not even know what is going on. Once they tell him on the train, Tomoya is dead set on going back home, and makes it to the train platform which would take him home, but anticipating this, Nagisa’s parents were waiting for him with Ushio. Tomoya’s friends block his escape route, and Tomoya turns back to see Ushio starting to run toward him. She trips midway which makes Tomoya leap out and catch her. Picking her up, he is able to see the continuation of his old dream once more, and sees Nagisa under the promised tree smiling lovingly at him and Ushio.