Story summary by Platina:
Anko 3301: The Hidden
A stray Marisa on her way home from hunting hears a Reimu calling for help inside a dark drainage ditch. She goes in and finds the weak, trapped Reimu, but is immediately attacked by a Flandre, who wounds Marisa and drags her away. Now that Reimu has served her purpose, Flan kills her and eats her bean paste.
Backstory: This Flandre grew up in the wild and came to town in search of better hunting grounds. She found the urban stray yukkuris to be easy pickings, but being unfamiliar with humans, she fell victim to an abusive mister who ripped her wings off. She took refuge in a covered drainage ditch.
No longer able to hunt and nearing starvation, Flan captured a curious Marisa who wandered into the ditch and said she'd let Marisa go if Marisa lured another yukkuri in to take her place. With no other choice, the first Marisa shouted about all the delicious sweet-sweets she had in the ditch, attracting a Reimu. However, instead of honoring her promise, Flan ate the Marisa and turned Reimu into her new hostage. In this way, Flan managed to continue to survive.
The time comes for the Marisa captured in the introduction to act as bait. Calling for help, she lures a Chen into the ditch, but Marisa begins laughing, saying that this will be Flan's downfall. Flan angrily kills Marisa, not realizing that this Chen is a pet yukkuri with a tracking-chip implant. Chen's owner rescues Chen and gives Flan to his neighbor to be punished. The neighbor is the same mister who tore Flan's wings off.
The abusive mister incapacitates Flan's legs, then sics a mob of angry yukkuris with sharp objects on her. He plans lots of other torments as well. He tells Flan that he'll heal her and let her go if she can lure another Flan to take her place. Flan calls for help, but unlike her childhood home in the mountains, it is unlikely that there are many other Flandres around to hear her.