Rainbow Dash has fought urges that have become so distracting that she's already crashed twice. While at the doctor's she finds out she's way overdue for an exam that she's only had once before and swore to herself she'd never take again. However while there, meets a mare who's having troubles of her own. They've never met before, but that doesn't mean they don't know each other...
Confused? So are they.
UPDATE: Rather than try to figure out/agree on how many moons/revolutions/etc. we've got between out two realms, my tales work from the premise that 16 years old in Equestria is 16 years old on earth. FYI - AJ is about 23, Twi is 22, Rarity is 22, Flutters is 20, Pinkie is 20 Dash is 19.