Setsuko coughs again and again. She asks Seita what is wrong. He explains to her that she needs to eat more or she will die. Seita gives her his rations of food. Seita continues to give Setsuko his rations for a few weeks. She begins to slowly get better. But, Seita begins to slowly get sicker and thiner from complete lack of food. He begins to cough and gets red spots all over his body like Setsuko had. She asks why he is getting sick with her cute little innocent voice. He explains that he is just getting sick and he will be okay soon.

Later that day Seita goes out to get some more food. Setsuko buries the fireflies from the previous night. Nighttime comes. Seita still isn't back. Setsuko decides to go look for him. She leaves the cave. She wanders around for hours on end. Then she reaches the farm Seita stole food from.

Seita is lying face down in the dirt. His breathing is slowing. He begins to gasp for breath. Setsuko asks him what is wrong. He tells her he is going to go to sleep now just like the fireflies do. Then he orders her to go back to their aunts house and see if she can live there with them. He pulls out a piece of paper from his pocket and tells her to give it to their aunt. She leaves and walks towards their aunts house.

Seita's breath gets heavier and heavier. His heartbeat slows down. He coughs. His voice is dry and empty. He tries to gets up but falls down. He lands face first into the dirt. His gasping gets quietier and quietier. Then, it stops completely. Seita lays motionless on the ground.

Setsuko goes to their aunts house and gives her the note. It explains that if she is reading this Seita is dead and Setsuko needs a place to stay. It asks her to take care of Setsuko. Her aunt says no.

Setsuko leaves the house and wanders back to the cave. She lays near the grave of fireflies she recently buried. She stays there for days. Not moving. Not eating. She slowly starves as the life leaves her. Her soul has already died along with Seita. She lays there for weeks until the grave of fireflies becomes the grave of Setsuko.

Fireflies continue to fly to the very cave where they lived. They drop there dead when there time is up. Setsuko's grave is littered with the dead fireflies. They have been forgotten by the world. No more then a helpless girl and a bunch of useless insects. That's what people say. But if you look out at the fireflies at night you can see in there light the joy of a family reunited. The memory of a boy pushing his sister on a swing. The hope for a brighter future. And the sadness of the truth of reality.