Holmes away from Home

On a plane from New York to London, two girls meet. They talk and find how weirdly similar they are, in everything including the castle school in Scotland that is their final destination, but a twist of events makes them realise that their bond runs deeper than they thought.

I DO NOT OWN MOST OF THESE BEAUTIFUL CHARACTERS! They belong to:

A) BBC
B) Sir Arthur Conan Doyle
C) J.K.Rowling
D) There are brief mentions of ones belonging to Rick Riordan, Robert Muchamore, and John Green