It's story about a pet Marisa who wanted to help other yukkuri, especially strays. But she understand her position as pet and her powerlessness.
One day, her owner suggested her to have children so she can direct that love to them. He injected her with lots of Alice's sperm paste and so Marisa had lots of children. But she can't take care all of the children and they ends up loving her owner more and treated Marisa like slave. Mister suggested Marisa could be a bodied yukkuri so she can help other yukkuri more. That does happened. Marisa, now bodied, is abandoned in a park near the mountain forest. Here, she met a stray Reimu who's street smart (she know the danger of giving children sweets sweets in long run). Despite being refused, Marisa helped Reimu's family by bringing cardboard. Under Reimu's direction, Marisa easily made the cardboard house. Marisa helped other stray yukkuri too, building them home, food and treasures. But Marisa is stupid so the newly formed clan don't like her very much. There's also episode when Marisa bathed the little ones and they ends up dead from mold growing on their head under their wet hairs. Marisa is entrusted as babysitter for the children because the stray clan's adults look at the baby as burden and shit machine. Marisa brought the children to play in pet yukkuri's playground. Pet owners troubled by Marisa's act (such as her feeding raw caterpillars to the koyukkuri) reported the matter and a total extermination is done. The leader Patchouli and Alice killed their children and committed suicide after saying good bye to stray Reimu who lived far from the clan. Marisa who realized she's the cause of the clan's destruction is depressed.
The whole clan is taken for processing. Marisa escaped while the stray Reimu and her child probably escape. Seeing the picture, maybe this is that pair.
The clan leader Patchouli know enough to escape. She know if she stayed like the rest, they will be brought to Factory and suffer for the rest of their life. That's why she killed her children (after failing to tell them to do Please Eat Up) and then split herself.
The clan's demise is partially her fault too. She felt indebted to Marisa who has provided the clan with house and food. This way, her husband Alice can stay at home and get pregnant with two children (if Patchouli, she wouldn't survive the pregnancy).
But well, the direct cause is Marisa. Being provided with food and shelter, actually the clan now hides inside the forest and never interact with human anymore.
BTW, from the signature at lower left, seems to be Kurumata's drawing.