The story starts here:
Yuyi the mermaid and her lover live together in the barn. At the start, everything is well and they’re happy together.
Their dream (at first hers too, but mostly his) is to get married, Yuyi becoming normal and human. The scene is very dreamy and keeps persisting through the video, as something that could have been, but actually is unreachable.
Things start to get sour at some point. Yuyi, used to live in the ocean, has to live in a bath tub now and it’s not really best conditions for a mermaid. She’s also in the constant care of her lover and depends on him.

(the expression of relief she has after getting water thrown on her, like she didn’t get it in a long time because her lover was getting tired of taking care of her)
Also, her lover wants to have sex with her, but he can’t because she’s a mermaid.
Growing tired of this, he decides to try and turn her into a human. At first, she seems to go along with it because she loves him.
His first experiment is the brunette Gaga:
She has legs now :
But she still can’t live properly outside water:
So he proceeds to make more experiments on her, trying to ‘fix’ her, ignorant that he’s torturing her for his own wish:
My theory is that, as he does all these to her, she finally realises he’s trying to change her and she doesn’t actually want that. In the scenes with Jo, she finds the love for herself again:
So she starts to rebel.
When he manages to ‘fix’ her, she has legs and can live out of water, but she doesn’t love him anymore, after what he put her through. So she leaves him: