The Church received intel about a magic cabal preparing a spiritual device of mass destruction on an island in the Micronesia. They sent Kanzaki in to completely destroy the device, stop the cabal's plan, and to kill or capture the mastermind behind it. As a result, Kanzaki, the jeans shop owner and the tour guide lady went to the Uphill Island (アップヒル島) to investigate and resolve the issue.
Uphill Island was originally just a chunk of rock sticking out of the ocean with only 3 meters in diameter. But 50 years ago, an underwater volcano eruption caused a plate movement, then the current Island was formed. Because of this, Uphill Island is seen as an irregular and a bad omen to the surrounding islands, and so they don't have a good relationship with each other. Also, the locals on Uphill Island have developed a belief that "people who live here are good, people who come from outside are bad" and generally do not like visitors, making investigation even harder.
After some investigation, it looks like that the local religion is influenced by the "cargo cult" religions. The locals think that the good things given to the surrounding islands were originally meant for the new island as a blessing from the Gods, but the other islands stole them. It's as if the locals are all preparing something to take those blessings back by force.
At first they suspected that the locals are all involved in this mass destruction magic plan, but this idea was dismissed quickly. Then they tried to figure out what religion that spiritual device is based on. It couldn't be the Island's modified version of cargo cult, since it's relatively new compared to the other branches on the other islands, and it doesn't have a record of large scale magic activity. It also couldn't be the main religion of the surrounding islands (believing in the blessings of the sea), since all the important artifacts of that religion are on the other islands, and this island does not have priests (or equivalent) of that religion. In the end, they figured that it's most likely something left on that 3 meter rock by Christians during the Age of Discovery, before the island was formed.
Later, Kanzaki arrived at that rock alone. It's in a part of the island with no settlements. There, she saw a 10-year-old girl whose name was never given appears to be another English magician doing something at the rock. When Kanzaki was about to fight, the girl invited her to come take a look at what's on the rock.
It's not something left by Christians, but rather, it's a set of runes. However, it doesn't have any magical meaning to it, since it's more like the Japanese "a i u e o" table. The girl explained that it's something related to Norse myth. During the Age of Discovery, Christianity was forced upon many nations. That set of runes was left by believers of the Norse religion on territory that was not yet covered by Christianity, in hopes to keep their religion alive.
The girl explained that she came to this island to investigate the impacts that her religion had in different cultures. After deducing that the girl is not a threat, and that there must be another magician on the island, Kanzaki gave the girl an amulet sort of thing for her to wear on her neck to track her position (so in case there is magic activity on the island, she can prove that she was not there) and to communicate with Kanzaki in case of an emergency. Then the girl suggested that the real culprit is someone who can cause a great flood that can wash everything, since this is probably what the locals want to happen because of the treatment they get from the surrounding islands.
Just as Kanzaki was about to leave the girl and go search for the real mastermind, the girl triggered a red flood which swallowed the part of the island that they are on, avoiding herself but washing Kanzaki away into sea. The water that washed her away is red and has an unnatural consistency, making it harder to move around in the red water. This spell is called Ymir's Ocean, which allows her to control the water and salt in the ocean. In order to use this, she'll have to change the salt concentration of her own blood to match that of the ocean's, since the ocean was created from Ymir's blood in Norse myth. However, if she does that, she'll definitely damage her own organs. So she concentrated all the salty blood in blood vessels in her hand, and stopped it from flowing to other parts of the body. Of course, this means she has to restrict the flow of her blood, so she cannot use this spell for a long duration.
After being washed away, Kanzaki reappeared on top of the water, on waterskis (she hid her sword in a waterski case, so she probably had real waterskis in there too). She explained that Ullr (a.k.a Ollerus), who is known as the god of hunting and skiing, was among the gods who were not swallowed by Ymir's flood. In other words, as long as she's on Skis, Ymir's flood will not sink her.
The girl then told Kanzaki the reason why she's doing this. Originally, she really did come here for research. She skimped on the people-clearing magic because she didn't expect people to come, and in order to prevent the sea water from eroding the rock, she used her magic to control the water current. However, she was seen controlling the water. Because of this, the locals think she is the key for the island to gain power, and the surrounding islands think that they are going to be swallowed by a flood. In this situation, if she leaves the island without doing anything, everyone on the island will definitely be killed by people of the surrounding islands. This is why she can't leave.
The girl then proceeded to create salt pillars to attack Kanzaki, but she dodged them all. When Kanzaki had the chance, she used the force of an emerging salt pillar and leaped into the air. Then she dodged a few more salt pillars in mid-air. After Kanzaki landed, the girl went full force and sent a large wave towards her. However, this pushed Kanzaki towards the island. Then Kanzaki headed straight for the girl while on skis and knocked her out with one hit.
After the girl regained consciousness, Kanzaki offered the girl to leave quietly with her. Then they hear loud cheers from different parts of the island. The locals are cheering because after seeing the red flood in action, they are confident that this power can help them destroy the enemy.
In the end, the girl decided to stay and try to resolve the issue without using the red flood.
- Ymir's Ocean - Ymir's Ocean Girl