The mother goes away and the oldest boy, Akira, takes care of the younger kids until she comes back. Then the mother goes away and doesn't come back, but just sends money. Less and less money and less and less often. Akira finds their father, who not only refuses money, but refuses that he's their father. The older girl clearly knows what's happening and is also trying to make things feel "normal" for the two younger kids.
Akira starts entering puberty and meets an outside girl whom he trusts enough to bring to the apartment. She clearly cares for them and helps when she can. Akira would like to work, but he's 12 now and you have to be 16 to work part-time. Things become rougher -- the power is turned off, the water, and the landlady shows up and asks for the rent (the kids say they're just cousins visiting) -- and the only good part is that an employee of a market gives Akira the out-dated food at the end of the day, out the backdoor and without the owner's knowledge, so they have some small amounts to eat.
One day Akira comes home from scavenging and finds that the younger girl has fallen from a chair and is cold. He has a strong emotional reaction -- he's taken so much care and now she's dead -- and then he and the outside girl put her in a suitcase (a bigger suitcase than the one she came in, she's grown) and take and bury the suitcase at the airport grounds.
The last time Akira talks to his mother about staying and more money for them, she says "Don't I get to have fun, too?" (subtitle) This is a heart-wrenching movie, but worth watching.