It starts off as a run, as you run away from Regigigas (Like the Porky scene) you have to break boxes and rocks out of the way from your running path as the demented giant chases you. After awhile, running away from him, a cut scene begins, showing Mario and Luigi being cornered after running. Just as Regigigas approaches, a giant, line of light appears, hitting him. The light lands in front of Mario and Luigi. Who is the light? Newcomer Victini!
So, playing only as Victini, you have to defeat Regigigas. He attacks with powerful, but slow, punches, stomps, and one grab attack: Crush Grip. If he grabs you, he crushes you like Master Hand does (Except you can see your head). You escape him by mashing the buttons on your controller (Like you would normally). After halfway through his HP bar, he'll glow brighter than before. Slow Start has lost its affect. Now the beast is angrier, faster, and stronger. His Crush Grip squishes more, as well. Upon being defeated, he lets out a deep roar.
After defeating Regigigas, he will see you as a friend and somehow assist you later in the story. He'll present you with a treasure chest. Mario and Luigi approach, and Victini turns, giving them the treasure inside. Mario smiles, then gives Luigi a high five and Victini a big hug. After all this, you will unlock Victini as a playable character.
Later in the Story, you see Regigigas giving you a literally BIG help that's as big as he is when you fight Hewdraw.