Okay Mummsy,

As I am also doing another novel to add to my series, I will try and help you solve your little problem.

Yes sequels are continuations of another book, but they are also their own story.

Example:
My first book in the Andreas Saga, started Tia on her journey to discover who she was. That was a sub-plot to the Saga's Plot but it was the Main Plot for Book 1 Tia's Mysteries

The Next Book in the Saga, "The Long Lost Princess" had Tia finding out more about her Father's and Mother's past. Again a Sub-Plot for the Saga but The Plot for book Two was her finding out her connection to Egypt.

This Third Book in the Saga, "Challenges of A Guardian" Has a Plot of it's own but that Plot is just another Sub-Plot to the Saga as A Whole

So, Mummsy: For Day 15: Think what is The Sub-plot for my series that this sequel is focused on? That is your Plot for that Book!

Hope this advice helps.