While Starlight Glimmer is in Canterlot for a few days, a couple of ponies from Starlight's past arrive in Ponyville to entertain the locals but after seeing who lives there, they decide to cause problems instead.
At the age of 20, Twilight Sparkle is still without her mark and since her brother is the captain of the royal guard, he could get Celestia to help her. Once Celestia realizes that she can't help her, she sends her to her protege who possibly can.
When Princess Celestia falls ill, she summons Starlight Glimmer to Canterlot to take over her duties for a few days. Celestia is confident in her protege's abilities but Luna has her doubts.