Sunset Shimmer was highly known ever since the Fall Formal incident. That is, until she unexpectedly commited suicide, via knife to the chest. They came to the conclusion that it was depression which lead her to take her own life away.

However, not everyone agrees.

Together, the rest of the Mane gather clues and information, in an effort to find the real reason behind her suicide.

That is, if it is suicide to begin with.