Hi there. Let's begin with a few assumptions about the world I'll be writing about.
First: These characters are all from the movies Frozen and Rise of the Guardians. They're not mine. This is purely fan made and not intended for profit. Promise.
Second: I understand that the world of Frozen is set in the 1840's, while the Guardians' is set in present day. To combat this and create a believable setting, I've decided that in this story Arendelle and the other island kingdoms are part of a historical experiment started after WWII. The people are in an isolated, no fly zone in Norway where their culture and interactions are studied by a group of scientists integrated into the villages. For the most part this information isn't important at all to the story, and simply serves as a backdrop to make it possible for Jack and Elsa to even meet in the first place.
Third: The royalty involved in this experiment are aware that the world outside their islands is different and dangerous, but none know the details beyond that. The experimenters let them in on it so they could be used to explain away any mistakes to their subjects.
Fourth: Let's assume that powers like Elsa's are extraordinary but not unheard of in the modern world, so she hasn't been swept up by scientists eager to study her.
That about covers it. This is my first work of fan fiction, and I'd greatly appreciate feedback of all kinds.
Happy reading!