First things first, we should start with a definition.

Cambridge dictionary determines the word “mystery” as “something strange or not known that has not yet been explained or understood”; and the word “interaction” as “an occasion when two or more people or things communicate with or react to each other”.

So that helps me to come up with a new definition:

Mystery interaction — an occasion of communication between two or more people or things with a strange or not yet explained outcome.

Cover source:
Arrival (2016) movie trailer