September 9, 1945. A teenage boy, clad in rags, sags against a column in a
train station. As passengers going past complain about the homeless in the
station, Seita slumps over and dies. The janitors who remove his body find a
rusted can, which they pitch into the grass outside. The can pops open,
spilling its contents. As fireflies in the grass begin to fly away, the spirit
of Seita’s younger sister Setsuko stands up. She looks to where her brother’s
body lies and starts toward him. Seita’s spirit appears beside her and puts
his hand on her shoulder. She turns to him and smiles; he smiles back. Seita
picks up the can, and the rust clears to reveal that it is a can of Sakuma’s
drop candy. He hands the can to Setsuko, who smiles. She takes him by the hand
and they walk off-screen as the opening credits begin.

In the backyard of his family’s house in Kobe, Seita buries food. He straps
Setsuko onto his back and begins to look around the house. He finds a picture
of his father in a Japanese Navy uniform, and takes it. The firebombs begin to
fall as Seita dons his boots. He and Setsuko are unhurt, but the neighborhood
begins to burn. Dodging flames, Seita and Setsuko find their way to the beach.
From there they can see the entire coastline in flames.

After the raid, they return to find that Kobe has burned. Setsuko complains
that she has something in her eye, so they go to a makeshift hospital at the
school. While a medic washes out Setsuko’s eyes, a friend rushes to Seita and
informs him that his mother has been hurt. Leaving Setsuko with the friend,
Seita enters the hospital and finds that his mother has been badly burned.
Seita returns to where Setsuko has been playing the schoolyard and tells her
that their mother has been hurt, but will recover.

The next day, after taking Setsuko to a distant aunt in Nishinomiya, Seita
returns to the school to find that his mother has died. Her body is burned in
a mass cremation. He returns to Nishinomiya with a box containing her ashes,
which he hides from his sister and aunt.

Seita returns to the city to recover the food he had hidden. He brings to food
to his aunt, who delights at the rare food items and comments that only
soldier’s families get such luxuries. When his Aunt suggests brining Setsuko
to visit her mother, Seita reluctantly tells her of his mother’s death.

On a rainy day, Setsuko plays indoors while Seita reads. Their aunt enters and
asks why Seita isn’t in school or at work. He replies that his school has
burned down and that the steelworks in which he was working was bombed. She
asks if he has written his father, and he replies that he wrote as soon as he
arrived.

Some days later, Seita takes Setsuko to the beach. They pass people making
salt from seawater, and Seita tells Setsuko that it is because salt has been
rationed. They splash around in the surf for a while, and then sit on the
beach eating candy drops. Seita begins to reminisce about pleasant trips to
the beach with Setsuko and their mother, but his reverie is cut short by an
air-raid siren.

When they return, the aunt suggests that Seita barter his mother’s kimono for
rice. He agrees. Setsuko overhears and begins to cry. Later, the aunt returns
with a sack of white rice, and they a rice dinner.

At breakfast, Seita and Setsuko eat rice soup while their aunt prepares
rice-ball lunches for her daughter and a boarder. Setsuko complains that she
doesn’t like the soup, and Seita tells her that they will have white rice for
lunch. The aunt tells him that lunch will be rice soup again, and complains
that Seita isn’t working. When Setsuko observes that the rice was theirs, the
aunt angrily suggests that they prepare separate meals. She then observes that
Seita and Setsuko have other relatives in Tokyo, and suggests that Seita write
to them.

Seita goes to the bank, where he finds that his mother has left them some
money. He buys a stove and some dishes. When they return, Seita cooks for the
two of them and they eat in their room. In the dining room, their aunt
complains to her daughter about how ungrateful Seita and Setsuko are.

When the aunt returns, she finds that Seita has left dirty dishes in the sink.
As she washes them, Setsuko awakens crying. The aunt enters their room and
complains angrily to Seita that her daughter and lodger need their sleep since
they work for good of the nation. Seita takes Setsuko for a walk among the
fireflies to put her back to sleep, and an air-raid begins. As they hurry to a
shelter dug into the side of a hill, the aunt’s voice-over chides Seita for
going to a shelter rather than helping put out fires.

Seita flashes back, and we see him entertaining Setsuko. Their aunt, working
outside, comes to the window and chastises Seita for playing a cheery tune on
the organ. She complains loudly that Seita is no help during the air-raids,
and adds that they can live in the shelter for all she cares.

Back in the shelter, Seita stirs from his recollection and suggests to Setsuko
that they should move into the shelter. He borrows a cart and loads their
possessions onto it. As they leave, Seita apologizes to his aunt for having
put her out for so long.

When they arrive at the shelter, Setsuko runs about excitedly while Seita
places the box containing their mother’s ashes under a bench. At dusk, Setsuko
complains that it is dark inside the shelter, so she and Seita catch fireflies
and bring them inside.

The next morning, Setsuko digs a mass grave for the fireflies. As she scoops
them in in handfuls, Seita recalls the mass cremation of their mother. Setsuko
tells Seita that she knows that their mother is dead because their aunt told
her. Seita begins to cry and promises to take Setsuko to visit their mother’s
grave.

Time passes. Some kids find the shelter where Seita and Setsuko have been
playing. One finds the grave of the fireflies. Another finds the food Seita
and Setsuko have been eating, and complains that nobody he knows would eat it.

While walking on the road, a fighter attempts to strafe Seita and Setsuko.
They dive under the cover of a field of ripe tomatoes. Seita is unable to
resist and steals some for himself and Setsuko.

While bathing Setsuko one day, Seita notices that she has lice and that her
entire back is now covered in a rash. They are both noticeably malnourished.
Setsuko tells Seita that she has been having diarrhea for some time.

Setsuko begins to loot unattended houses during the air raids. He steals some
kimono, which he tries to barter for food, but he has trouble finding a buyer.
When he returns, Setsuko is passed out on the grass. He rouses her, but she is
delirious, so he takes her to see a doctor, who tells him that she is
suffering from malnutrition. He asks for medicine, but the doctor tells him
that all she needs is good food.

Seita withdraws the last of his mother’s money from a bank. While walking away
from the teller window, he overhears that Japan has surrendered and learns
that the entire Japanese Navy has been sunk.

When he returns to the shelter, Setsuko is lying on the ground, emaciated and
delirious. He brings out a watermelon that he has purchased, cuts some, and
feeds it to her. She falls asleep, and he goes outside to cook a meal. A
voice-over tells us that she never wakes.

The next day, he buys charcoal for her cremation. As he walks back to the
shelter, we see Setsuko’s spirit playing outside while music plays on a record
player in the distance. Seita places Setsuko’s body in a wicker coffin with
her favorite things, takes it up to the top of a hill overlooking the shelter,
and burns it. When the ash cools, he puts some of it into the candy tin.

As the film ends, the image of Seita sitting on a bench atop the hill
dissolves in to an image of the spirits of Seita and Setsuko on the same
bench. Setsuko falls asleep with her head on her brother’s lap.