Legend tells of a mysterious fog and its giving nature, but what happens when that fog comes around to Ponyville? When Pinkie Pie encounters the one behind the fog, they demand that she teach what it is like to feel joy. Pinkie informs Twilight of this and together they're reminded what joy is. How the smallest spark can change a bad day to a good day. They also learn why the one behind the fog brings about good luck and why they ask about joy.