E4, real name Erin, awakens in a body not her own in a world of Myst. Taking up the guise of a maid and pretending to be her body's previous identity she is swiftly introduced to life as a Regulator. Her job is simple, travel the rails and slay monsters that would threaten civilization.

Yet E4 has secrets and enemies, not all of which were left behind. For the Mysts draw monsters and nightmares from the three, or four, Earths.

And E4 comes from a time of biological nightmares.