The year is 157 of Friendship Era.
Equestria has entered into the second golden age.
The time traveling spell had been worked on, and improved so much that traveling time has been increased from few minutes to almost infinity.
They say there is a castle in abandoned city.
In it you could find a well preserved Alicorn's skeleton.
So far cause of death is 2 inch hole drilled right into center of a skull.
Nopony knows who did it and why, Alicorn's identity is also unknown.
With Princess Twilight blessing time traveling agency organized an investigation.
The idea was to send a pony into the past, keep low profile and observe.

One mare volunteered, can she do it without getting caught?