This manga starts off a little slow and hard to get into, and kinda leaves you hanging at the end of it. The story follows two girls Yamada Nagisa and Umino Mokuzu who become friends through somewhat of a strange situation.
It starts off with Nagisa walking up a mountain path talking about something her brother had once told her. Then it cuts into her being in the classroom looking bored. At this point in her life she's made up her mind that once she's graduated from Junior High, she's going to join the Self Defense Force Camp instead of going into High School.
Since her dad died 10 years prior in a typhoon her mom has had to work two jobs just to make ends meet. They also relied on her father's insurance money until it ran out and that's when she decided to join the Self Defence Force. She also a brother who is a few years older than her, but about three years ago a girl came to the house and said something to him and he became a shut-in. He dropped out of High School and hasn't left the house since then. Every day after school she goes home to cook food for her and her brother.
One day, there was a transfer student, Mokuzu, that came into her class. At first she was really shy until she took a really big drink of water. This is when she declared that she was a mermaid. Mokuzu decided that she was going to have Nagisa as a friend. She did everything in her power to maker her, her friend. The first thing she did was throw a water bottle at Nagisa (I thought this was funny, because it kept happening). There was a rumor that her father is this big famous artist/ celebrity.
Eventually Mokuzu and Nagisa become friends but their friendship is that of a strange relationship. It's not that of a normal friendship. Mokuzu is always clinging to Nagisa, trying to become more of a friend to her and trying to get her to believe that she is in fact a Mermaid. Nagisa is always asking and talking to her brother about Mokuzu. Her brother once tells her that she believes in "bullets" that Mokuzu is firing "sweet bullets" at her.
She also keeps telling her that there's a storm coming, one that comes only every 10 years. The one that took her father 10 years ago is coming again, when all the mermaids are coming back to the Sea of Japan to lay their eggs. She also hits a sore spot when she talks about her father, when he died at sea telling her that he was at the bottom of the sea with a lot of mermaids and gold and very happy.
As the story develops their friendship becomes more solid and they end up making plans to run away together to find "safety" for children. After Nagisa packs they go to Mokuzu's house where Nagisa waits outside for a couple hours for Mokuzu to come out. Instead of Mokuzu, her father comes out with a suitcase in tow. So Nagisa enters the home to try and find Nagisa and she comes across a scene that she's seen once before. Mokuzu's father catches Nagisa in the home and asks why she's there. She ends up telling him that she's looking for Mokuzu. Her father lies to her at first. Then he tells her that Mokuzu has turned into "Sea Foam." Which is something that Mokuzu mentioned earlier on in the story.
Nagisa is in a daze as she goes home, and heads straight to her brothers room. At first she's so stunned and try's to get out that Mokuzu's father has killed her, then when she finally gets it out she collapses into her brother's arms. Her teacher happens to be there to talk about her choice for after she graduates Jr. High. Her mom and teacher rush in and totally deny the fact that this is something that is real. Her brother is the only one that believes her. So in the early hours of the morning of 4 October her brother and her set out to the mountain to where she's most likely at and find her chopped up into pieces. They then go to the police and tell them what they find.
The story ends with Nagisa returning to school and Mokuzu's father being arrested. Her father confessed to everything that he had done to her.