Ah, yes. The movie of rumor and educated quesses. There is no real concrete information on the 4th movie other than it will exist. The Pokémon seen above is Serebii, number 251(also spelt Cerebii). As for what's sapose to happen in this unwriten movie, that is still under raps. Some think it will be the last pkmn movie, hope not. Personality, I know what I wish would happen in the movie, or atleast in the beginning. Ahem, it would start something like this: Narrator: "Many trainers have wonderful experiences while training but none like what Ash Ketchum has gone through. Before our feature, we'll recap how it all started. That is, his first truely great adventure. And it all started with a Pokémon named Mewtwo..." Yada yada, we recap almost ALL of Mewtwo Strikes Back some of Revelation Lugia then some of Lord Of The Unknown Tower. Alright, alright, none of that's gonna' happen, but don't you wish? Anyway the only real info is that Ash opens the G/S ball, releasing Serebii. Anything after that is in the works over in Japan and/or Nintendo ain't talkin' yet.
