After killing Deadriser, Sargast looses his mind completely, and ends up in Canterlot's mental facility. However before Celestia, Luna, or any of the elements of harmony could get back to the mental facility, Sargast has escaped and is now lost somewhere in Equestria, and Luna and the elements of harmony must find him before anything happens to him, or Equestria.

Multi-point of view aka it switches between Sargast and Luna/Elements of harmony.