Yuta Ito receives a smartphone for his birthday. Shortly after opening his present, Yuta's terminally ill mother assigns him the task of filming her and compiling a movie about her in the event of her death. After she dies, Yuta premieres the movie at his school but is met with heavy derision over his decision to end the film with him running away from an exploding hospital. Bullied and ostracized, Yuta decides to commit suicide by jumping from the roof of his mother's hospital. He's stopped by a girl named Eri, who reveals she actually loved his movie and urges him to make another one. The two work together to bring this to fruition, alternating between production and marathoning various movies for inspiration and education. They decide on making the movie a semi-documentary about themselves, but with various exaggeration and fictional elements, most prominently the idea of Eri being a vampire.
Yuta and Eri grow closer as time goes by until Eri falls unconscious while the two were playing at a beach. Eri is revealed to also be terminally ill like Yuta's mother. A depressed Yuta is discouraged from attempting any more progress on his movie. However, his father encourages him to push forward, while also revealing to the reader that Yuta's mother was actually abusive towards her son and husband. The film she had Yuta make was an attempt to either capitalize on her illness if she survived, or memorialize her in a positive light if she died.
Yuta and Eri complete their movie, not long before Eri dies. Yuta screens his new movie, which is met with praise this time around. Yuta is later confronted by one of Eri's friends, and the two admit that Eri was not the idealized version Yuta filmed and edited for their movie, but both agree they prefer to remember their mutual friend this way.
Since then, Yuta spends time balancing his normal life with his obsession to recut Eri's movie. Years later, an adult Yuta suffers the loss of his wife, child, and father in a car accident. Losing the will to live, Yuta had decided once more to attempt suicide at the screening room where he and Eri would marathon movies. Arriving there, Yuta discovers a still alive and youthful Eri. Eri reveals that she really is a vampire who repeatedly experiences memory loss caused by cycles of brain death throughout her eternal life. The Eri from her previous life had left specific instructions for her future incarnations as well as the movie she and Yuta made to ensure that she will remember him forever and know what kind of person she should strive to be. His will to live restored, Yuta bids farewell to Eri and nonchalantly leaves as the building the screening room was housed in explodes much like in the ending to his movie about his mother, leaving the reality of previous scenes ambiguous.