Yuu, a young healer with the power to heal the hearts of monsters, receives a mysterious bottle from his father as a gift. When Yuu touches the bottle, Merc, a girl made of liquid, appears from it. Merc has no memories from before she met Yuu and wants to learn more about who she is, so the two of them set out on a journey through various countries. And thus begins the comical fantasy of Yuu, an apprentice healer with a fear of monsters, and Merc, the girl in the bottle.