When a strange, unexpected storm settles over Ponyville, Twillight Sparkle investigates. She discovers that not only is the storm the fault of the weather ponies, but only the weakest flyer in Ponyville can stop it.

Forbidden magic is only forbidden when somebody explicitly forbids you to do it. This does not preclude possible consequences from engaging in poor magic practices.

Twilight Sparkle gets distracted in the middle of dabbling in questionable magics and things go downhill from there.