If you remember correctly, in the original Kingdom Hearts, you helped Ariel rescue the King's Triton from Ursula as you searched throughout Atlantica for her. In Kingdom Hearts 2, the storyline for Atlantica will reflect off of "The Little Mermaid", as we see the appearance of Prince Eric.

It seems as if Ariel has not learned to not trust Ursula in Kingdom Hearts 2 either, as she lets Ursula help her turn into a human, in order to meet Prince Eric, who lives on the surface. This allows Ursula to become one step closer into taking over Atlantica.

This is where Sora and the gang come in. They must help defeat Ursula in order to protect Atlantica. Rather than a battle like in the original Kingdom Hearts, Sora and the gang must face ursula in a musical mini-game!