They say look before you leap and make sure you can swim before you go in the deep water, but when a picture of his late grandfather falls into the ocean, Shima jumps in after it without thinking. Nearly drowning as a result, he is instead saved by a very perfect stranger... one whose strangeness extends to only being human from the waist up! For Shima, who's always felt like a fish out of water himself, it's more than just a revelation, and the young man and merman quickly begin to bond in ways neither anticipated. And yet, it's going to be far from easy sailing. After all, Shima and Isaki aren't just from opposite sides of the tracks, they're from entirely divergent species, and swimming in separate gene pools may make maintaining a long term relationship a whole different kettle of fish!

The story revolves around high school boy Ayumu Tamari, who has an illness that makes him start to turn to stone when he's stressed. Since he couldn't fit in with his class, after repeating a year, he declared that he wanted to have a "Sparkly youth" as well. To do this, he changed his hair and clothes to be more fashionable, and constantly checked for popular topics to stay in the know. In the midst of Ayumu's life of making a facade, his homeroom teacher and stone-loving geology teacher Kouya Onihara says that Ayumu's stone transformation is beautiful. Ayumu begins to become attracted to Kouya, who starts to give him advice.

When unstoppable aliens invade, it seems that all is lost until a teenager with the ability to defeat these invaders appears, but he has no memories of who he is or how to use his power.