Hey, I've read this one before!
A (relatively well-mannered) stray Reimu and her child Marisa wait outside the train station every day, begging the passers-by to adopt them. After a commuter annoyed by their noise kicks them around, another Mister takes pity on them and stops to help them. He asks about the scars on Reimu's back, and she explains that she was once the pet of a sadistic owner who abused her in various ways. Unable to stand it any longer, she broke a window with an ashtray and escaped. She started a family with a stray Marisa, but Marisa and all but one of their children were killed during an extermination. The kind Mister says he can't adopt them, but he gives them some candy before he leaves.
That night, Reimu and little Marisa think how nice it would be to be the pets of that Mister, and they figure that if they let him take it easy, then he'll adopt them. The way they'll let Mister take it easy is by making a boxed lunch for him. They re-use the plastic container from an expired convenience-store box lunch that they had scavenged and eaten, and fill it with lots of lovingly-gathered treats--balls of grass that Reimu has painstakingly chewed up and licked into shape, various insects, nuts and flowers, rotting vegetable scraps from the garbage, and for dessert, a popsicle stick with a lingering taste of sweetness on it. Then, they go back to the train station to wait for Mister to come by again.
At last, Reimu spots Mister. She tells him that she made him a boxed lunch to thank him for his kindness. Mister is puzzled. Just as humans can't tell yukkuris apart, Reimu thinks this is the kind Mister from the day before just because he's a man in his 20s, with roughly the same build, wearing the same color of suit.
Reimu proudly gives a detailed description of the menu, pointing with her tongue. Accosted by two filthy stray yukkuris, being presented with a box of garbage, and being expected to adopt the yukkuris, Mister snaps. The yukkuris wail and cry as he stomps on their food, then he starts kicking Reimu. Desperate to make him stop, little Marisa offers to give Mister her treasure, a bottle cap, but he takes her hat instead. To get it back, he makes them sing and dance, then does nothing but insult them, wounding their pride. Next, he humiliates Reimu by making her poop in the lunch box at his command (see picture), and then, holding a lighter up to Marisa's hat, he tells Reimu to eat her own poop. Reimu can't bring herself to do it, so Mister pushes her face into it with his foot, making her eat it, crying and retching. Mister laughs. Finally, he says that he'll make them his pets and feed them nice easy lunches--of yukkuri poop.
Mister happily takes them home--they've achieved their goal of becoming pet yukkuris.