In a world very much like our own, a chemical has been discovered that is going to test what two sisters know about reality. Are they Lucy and Sophia, or Luna and Celestia?

Why are they bombarded by memories of a world that cannot exist? How are they going to survive living a double life, and what can they do to fix everything?

It's worth noting that this takes place in an alternate human universe where MLP is not a show that exists. So the characters don't have the benefit of that 'reference'.