A possible concise summary is:
A girl helps a mermaid who has a crush on a human boy by giving her legs with a magic trident. However, they have to deal with a shark man who wants the mermaid as his bride and a storm that threatens their lives. The boy eventually realizes the mermaid's true identity and falls in love with her.
Here are the key facts extracted from the text:
1. The text is a transcript of a video about a mermaid who falls in love with a human.
2. The mermaid's name is Casey and she is helped by a human girl and another mermaid to meet the human boy.
3. The human girl uses a magic trident from a shark man to give Casey legs, but the shark man wants Casey as his bride.
4. The human boy saves Casey from the shark man and realizes that she is a mermaid.
5. The trident makes music when it is hit.