This artist drawing style is a little hard to follow, so I'll try to give a short summary of what I understood from images.
There's a yukkuri six-pack discarded into the trash (like food-grade yukkuri you would buy for your pet Remilia). Most of it's inhabitants either used or died in process of discarding, one koyukkuri Reimyu still survives and is "born" into the trash.
There she is discovered by a couple of stray Marisas "hunting" that trash. They promptly proceed to damage her accessory and designate her as poo-poo slave.
Later those Marisas fall victim to poisoned cake trap for strays, while shit-eater Reimyu survives and is freed.
Timeskip, she grows up into Reimu and finds a Marisa that has lost her hat and uses plastic bag for substitute. Damaged ribbon and bag hat is a match made in heavens, so they proceed to breed like cockroaches.
Soon after Marisa falls asleep, our Reimu awakens into fine Deibu and squashes all little Marichas - all but one that was sleeping inside daddy's bag hat.
Later when now Deibu is either distracted or asleep, Marisa calls out one Reimyu, and proceeds to squash her into fine pulp. Squashed Reimyu's ribbon goes onto survivor Maricha's head and now she can get out of hiding in the plastic bag. Maricha's hat is remaining on daddy's head under the bag, giving daddy a bit of comfort.
Koyukkuris (including disguised Maricha) proceed to happily eat squashed Reimu and wander away.
Again timeskip I guess, it's now winter and they have food, but thermal isolation is inadequate. They are doing yukkuri pile, but it's not helping much.
Some koyukkuris climb onto parent's heads. Then they proceed to pee in the sleep, with pee freezing into ice and stucking everyone together and to same place.
Next panel is either a dream or a flashback, I guess.
And final panel is happy end, thanks to mister cold.
Overall a nice story, but I would have preferred to see some more development into disguised Maricha story, in the end she did not matter at all.