Rainbow Dash finally rid herself of the curse that prevented her from ever ejaculating.
However, the only way to do so was to shift the curse to somepony else, and Pinkie Pie shouldered the burden for her.

One of the downsides to this is that now Pinkie Pie suffers from the same curse that Rainbow Dash had for years. In their search for the creator of the curse to discover a permanent cure, Pinkie Pie's hormones begin going out of control!
Now if only they could keep her from humping everything in sight long enough to actually reach their goal.