A wandering adventurer named Cain passes through an area known as the “Dragon’s Lair.”
After saving an injured old man from a monster attack, Cain learns that the monsters from the Dragon’s Lair have started attacking people. The old man wants to calm them down.
The old man possesses a Charm Crystal, an item that can be used to charm and control monster girls weakened in battle. If the monster girls can be controlled, the monsters should settle down.
While the old man recovers from his injuries, Cain takes on the task of subduing the monsters and is entrusted with the Charm Crystal. To ensure Cain doesn’t run off with the crystal—a keepsake from her late parents—the old man’s granddaughter, Miria, decides to accompany him.
And so begins Cain’s merciless “punishment” of the monster girls…

[Translated from dlsite]