Origins of Mega Evolution!

Finally arriving in Shalour City, Ash & co. meet up with their old friend, Korrina and her Lucario. While there, they had towards the Tower of Mastery where Ash challenges Korrina to a gym battle, despite not having mastered the techniques Tierno showed him. However, the night before the gym match, Team Rocket appear in the tower and steal a document dedicated to Mega Evolution. Will Ash & Co. be able to retrieve the document in time for the battle?