After being cured by Twilight's spell and creating the bat sanctuary in the apple orchard, Fluttershy learns that she still has some bat blood inside her. And that the bat side of her is thirsty. So thirsty...
Applejack has been crushing on Rainbow Dash for quite some time, and since Sweet Apple Acres is hosting a small Hearth's Warming party for her friends, she decides to make her move.