The sequel to The AppleDash Project, but that was episodic you can jump right in! All you need to know is:

Applejack and Rainbow Dash have been together for more than a year now. Their relationship has blossomed through Discord, a changeling army, and a whole hoof full of crazy situations and dumb mistakes. They've managed to form a partnership and learned to work out their problems together, not an easy lesson for two tough, stubborn ponies.

But now Rainbow Dash is convinced that their lives are getting boring, and if there's one thing Dash doesn't like, it's boring. So Dash has set out to make her relationship with Applejack more interesting. . . and AJ's not so sure she likes that sound of that.

From the original AppleDash Project:

The AppleDash Project is a series of short fics attached to various episodes, detailing a relationship between Rainbow Dash and Applejack behind the scenes. The stories take place sometime during or shortly after the indicated episode.

This is not meant to start shipping wars, argue that AppleDash is canon, or offend other shippers. In fact, if you like the idea I encourage you to try it with other ships!

Teen rating means that there may be talking about sex, but nothing on screen. Expect sitcom-like innuendo.