Mewtwo is fighting Genesect.
Genesect kicks his ass somehow because MODERN TECHNOLOGY VS OBSOLETE TECHNOLOGY
As Mewtwo falls unconscious from the fight, a voice echoes in his head
"Don't quit!"
Suddenly the gang all say "Look at that!" and "Who is that?"
Mewtwo opens his eyes, sees a strange figure fighting the Genesect
It blocks an attack, and mid block it looks to Mewtwo
It has his face; Mewtwo has a child
The Genesect deals a devastating blow, throwing the child into the gang
Mewtwo catches him with psychic power, then goes into a fit of rage
Obliterates a building, pushes Genesect to the brink of irreparable damage
Child rushes in front of the mortally wounded Genesect, and says "Enough! He's had enough!"
The Genesect is moved by this kindness, and befriends the child
Ash doesn't remember Mewtwo, but feels like he knew him at one point.
Bam, calling it now. Fucking Godzilla Jr mode.