Marin sees a girl throwing away love and decides, without being asked, that she's going to fix it. What starts as two mermaids playing matchmaker becomes something darker when Urin breaks a seal that was holding back actual consequences. The show doesn't pretend helping people is simple, and it doesn't let you forget that interfering in someone else's heart, even with the best intentions, can crack the world open.

Marin and her younger sister Urin live beneath the waves, and when a lovely silver ring sinks into the sea, they leave their home for the first time to return it. The owner turns out to be Kanon Miyamori, a girl who threw the ring away after her boyfriend broke up with her, and she tosses it into the forest again. Convinced that Kanon still loves him, Marin sets out to find it, but Urin wanders off and breaks a seal near a shrine, freeing an evil being called Sedna. A turtle elder named Matsumoto then reveals that Marin is the Priestess of the Sea, able to seal Sedna away alongside the Priestess of the Sky, a role Kanon soon learns is hers. The result is a magical girl fantasy with drama and romance.