It could be that she reads his book secretly. As she reads, she realizes that the book reflects the boy's thoughts and dreams, which makes her understand him better.
If 'cock' is a rooster, perhaps the plot could be that he found an abandoned rooster egg. He decided to take care of it until it hatched. He had to figure out how to keep it warm, what to feed the newly hatched chick. And as the rooster grew, they became good friends.
Perhaps it focuses on Harry's relationships. For example, his in - depth feelings for his friends. He might tell how Ron's loyalty affected him during tough times or Hermione's intelligence guided him. It could also be about his relationship with Ginny, how they really fell in love and overcame the challenges together.
A possible plot could be that the boatman and his wife have been living a simple life on the boat. Then the boatman meets a man (referred to as 'his husband' in a non - traditional sense here) who has great knowledge about sailing in different waters. The boatman wants to learn from him, which makes his wife a bit jealous at first. But as time goes on, they all realize they can work together. They embark on a long journey across the ocean, facing storms and pirates, and their relationship evolves into a strong bond of friendship and mutual respect.
His big smile might be very contagious. When he smiles, it just makes her feel happy and positive too.
It could be a story about a man who is somehow magically or scientifically transformed by a woman, and as a result, starts to experience things related to women's hormones. Like having mood swings or different physical sensations.
Perhaps she has a past trauma that makes her uncomfortable in the light. Dancing in the dark becomes her way of coping. In the fanfiction, she might meet a kindred spirit who also has a connection to the night and dancing. They could start to explore their shared love for this nocturnal activity together, and through their relationship, she begins to heal from her past.
In a fanfiction where Naruto is in the Bingo Book, it could mean that the ninja world's perception of him has drastically changed. Maybe he's been framed for a serious crime he didn't commit. This would lead to him trying to clear his name while also avoiding those who are after the bounty on his head. He'd have to rely on his friends more than ever, and there would be a lot of tension between him and the ninja villages that are now after him.
It could be that he wanted to keep the book safe and thought your bag was a good hiding place for a short while. For example, if he was about to go into a situation where he couldn't carry the book easily, like a sports activity.
One possible plot could be a curse placed upon him by an evil sorcerer. Maybe he offended the sorcerer in some way, and as revenge, the sorcerer uses dark magic to transform him into a woman. He then has to deal with all the new physical and social aspects of being a woman while trying to find a way to break the curse.