There was a Barbie who was a talented dancer. She practiced every day in her dance studio at home. One night, there was a big dance competition in the city. Barbie entered the competition with her best dance moves. She was so confident and graceful on the stage. The judges were amazed by her performance. In the end, Barbie won the first prize and became a famous dancer in the Barbie world.
A simple plot could be that Barbie is preparing for a party. She has to choose the perfect dress, do her hair and makeup, and also prepare some delicious snacks for her guests. It's all about the excitement and little challenges she faces while getting ready for the party.
Alexis might confront Beckett directly. She could say something like 'I think you're cheating on me. What's going on?' This is a very straightforward approach.
I don't know who Alexis is in this story without reading it. But it's likely that Alexis is either the main character or an important figure in the events that unfold in the 'creepy subway' setting.
A lawyer could be an important character. This lawyer takes on Alexis's case, believing in their innocence. The lawyer has to battle a corrupt legal system or powerful opponents who want to see Alexis locked up for good. The fanfic could show the lawyer's strategies and how they build a case in Alexis's favor.
Perhaps there have been some rumors or whispers in their fictional world that have reached Alexis' ears. Even if they are baseless, they can plant the seed of doubt in Alexis' mind, making her think that Kate is cheating.
She could be defensive of Beckett. Since Alexis probably has a good relationship with Beckett, she may try to convince Castle that he's wrong and that Beckett didn't mean any harm. For example, she might tell Castle about all the positive things Beckett has done for her.
Kate might get defensive right away. She could say things like 'How could you think that of me?' and be angry at Alexis for not trusting her. This is a common reaction when someone is wrongly accused in a fanfic or in real life.