
Okay, so
The boy lost his memory as a kid when he had an accident where he was hit by a vehicle. But, it wasn't really an accident because he was pushed by somebody and in later chapters we find out it's a guy with blonde hair that he bumped into one time at the beach. It turns out he's some creature that used to live in the depths of the Mermaid's pond and the boy saw him once when he fell into the water. I suppose the guy wanted to keep the "secret of the pond" from the boy and was scared that there would not be any more of those "openings" (so to say because I'm still not sure what's that in the deepest part of the pond) in water because you need mermaids to make them and the Mermaid wouldn't want to leave the pond if the boy is still there.
The boy forgot about the Mermaid as well and hasn't visited her not even once when we would visit the people in his hometown, and that for 10 fricking years... During that time, his childhood friend (that once spied in the boy and found out abou the Mermaid) would go to visit the Mermaid, trying to make her come out of the pond and talk with him. But, ofc, she didn't and was still hoping to see the boy again.
So, shortly, the boy somehow remembers the Mermaid and starts hanging out with her every day, teaching her how to read etc. amd realizing he's in love with her. She falls in love with him as well, they kiss, it's all romantic and everything; except the fact that the boy's childhood friend liked the Mermaid as well and is now angry at the boy for not telling anybody about his condition (where he needs to take notes of everything he does so he doesn't forget about it/can remind himself), especially for not telling the Mermaid that's going to be hurt if he just forgets about her one day and leaves again.
One time, the boy forgets lots of stuff that have happened and thinks he's a young boy again, visits the Mermaid, she gets freaked out by his behavior, he leaves in panic when he realizes what happened. The Mermaid, afraid that the boy might not come back or might just forget her and leave, asks the Pond Dude with blonde hair to give her legs so that she can go and meet the boy. It's all beautiful, they go to the sunflower field that the boy promised to take her to one day, buuuut, he forgets what's happening at some point and is guided (together with the Mermaid in his arms) back home by his childhood friend.
The end comes with the boy leaving the city, the Mermaid (after being taught how to read) getting the book about their love story (from the childhood friend) and kind of a promise to meet again when he, well, figures things out and "fixes" himself.
I apologize for such a long reply, but I suck at summarizing things, haha. But that's pretty much it.
I just finished reading it, and, booy.. That was ~kind of stressful~...