Ok, so this isn't going to be a complete summary of the story but just what you need to know for Kidnapped.

Lydia and Blair who are twins (Lydia being the younger) were trained by Sensei Wu to be ninja without the others knowing. Lydia was the purple ninja of darkness and Blair was the yellow ninja of light. There was a prophecy that Lydia would rise up and become the queen of darkness and consume all of Ninjago in darkness so Wu thought that, if he trained them as ninja and to fight evil, the prophecy would not be fulfilled.

The twins joined the ninja and eventually found out about the prophecy at the Temple of Darkness which caused Lydia to run away. The darkness in her soon overpowered and began to control her and lead her to the Temple of Light (It's not the same one as in the show and has nothing to do with that one. That was a mistake on my part because I forgot all about it).

Lydia consumed the temple in darkness and nearly all of Ninjago until the ninja came. Lydia was going to be destroyed by the darkness in her but Blair traded her light for Lydia's darkness, killing herself instead and giving Lydia light powers.

At Blair's funeral, Lydia reunited with her dad and you saw a bit of a crush forming between Lydia and Lloyd.

As for Lydia and Blair's backstory: They lived with their parents but their mom was much more favourable of Blair than Lydia. I don't quite remember at what particular age but it's not important that the twin's parents abandoned them. It was somewhere around twelve, but like I said, it's not too important.

Alright, I think that's all you will really need to know for Kidnapped, but if I left something important out, feel free to let me know. God bless!