This story is a sequel to Equestria Girls: Open Arms
The night before Aita's eighth birthday, his first in Equestria, his DS attempts to notify him about it. However, the combination of magic, rapid eye movement, and electricity unexpectedly brings the former changeling queen to the waking world, someone who was meant to stay in the dream realm.
Most of the teen rating comes from Moon Dancer's first line, and Twilight's highly technical explanation.