Apple Bloom, Sweetie Belle and Scootaloo are helpful fillies that aren't always entirely helpful. They are sick and tired of being on the sidelines while the big heroes save the day all the damn time!
However, they find a power that will allow them to overcome opposition and finally be helpful to the heroes! If they can learn to master it, so much the better. The group that they inadvertently join can help them to do so. For a price, of sorts...
Crossover with Centurions: Power Extreme