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 somepony who could help her. There is just one catch, that pony just happens to be Celestia's protege, Starlight Glimmer.

While this story takes place in the Glimmerverse, it is not canon to it nor does it affect anything that will happen in the future.