Batsy Fluffentuft (not the most respected of the lunar guards, and in fact on super-probation from Princess Luna herself) manages to escape into the town of Canterlot and have herself an adventure.
After a sudden realization about how Shining Armor became a Captain in the Royal Guard, Rainbow Dash hatches an insidious plot to become Captian of the Wonderbolts by following in his hoofsteps... by sleeping with a Princess, of course!
Ditzy is only lacking one thing in her life—aside from good balance, eternal wealth, princess-hood, etc. But since she can't do anything about the other items on the list, she is going to set her goals on something much more achievable.