A young filly appears in Applejack's life. It wouldn't matter that much if that young filly didn't pretend to be Applejack's daughter. Applejack tries to deal with her own emotions while facing that young filly, when Twilight tries to figure out who the filly is and where her real parents are.
But some other ponies begin to be implicated in a story that is, in fact, way simpler that it could seem at first sight.

Just a thought that crossed everypony's and everybody's mind. A story about what we all want.
And how hard it is to get it.