Arceus, after creating the world and the other legendary pokemon to maintain it, felt very confident that it would be able to run itself without incident. Feeling that it was most likely dangerous for one single entity - him - to have absolute control of the world, lest he one day go out of control, decided to step down from his position as 'god' of the pokemon world. Taking the form of a female pikachu, Arceus wiped himself of his memories and decended to earth, to live as a normal pokemon, leaving his legendaries to run the world and settle disputes.
For quite some time, things were peaceful, until it became obvious that Arceus had made a large mistake: He had not removed his powers from his new body, thinking that without knowledge of them they would go unused.
He was wrong.
Arceus, or Megumi, as she was now called, slowly began to grow bored of the melancholy of her world. Even after joining an exploration team, things still were not exciting enough for her. She found herself doing the same boring types of missions over and over again, and went from team to team searching for one that was out-of-the-ordinary, different, and interesting. She found no such team. Eventually, her desire for interesting events to happen awakened her dormant powers within her, unintentionally altering reality itself. The Deoxys, an alien race of pokemon, noticed an unusual explosion of data from planet earth, finding Megumi at the center. Celebi found herself unable to travel further back in time prior to the time of the data explosion, and Dalgia could see no physical evedince of time even existing before that moment. Several psychic-type pokemon were awakened to the existence of negative patches of space, with large titans dwelling within them (the ??? form of Arceus) and Palkia became aware that the spacial planes of their world and another were beginning to overlap, calling in beings from another reality, claiming to be something called "human".
The Legendary pokemon were very concerned with this situation. It was evident that if Megumi became displeased with the world to an extreme extent, reality could very well deconstruct itself and create a new world. Most legends agreed that they very much liked their current universe, and so they set out to observe the situation, attempt to prevent the displeasure of their unknowing 'god' and keep reality the way it was, and possibly repair the damage that had already been done. Other legends however, the more malicious ones, saw this as their chance to abuse the situation and gain absolute control of the world.
Megumi, completely unaware that she is causing the world to fall apart, continues to live her life to the extent that she would concider normal, wishing to find something extra-ordinary in her world, not realizing that the group of pokemon who were about to become accuainted with her were far from normal...