While flying between worlds, Aqua spots Terra, so she decides to follow him. Doing so causes her to end up in a world called Radiant Garden, where she meets Scrooge McDuck, who tells her that Terra went to the castle. In front of the castle, Aqua sees a young girl named Kairi under attack by the Unversed. She runs over to Aqua, who senses that her heart is filled with pure light just like the princesses. Mickey appears and holds the Unversed off while Aqua takes Kairi to safety. After getting her away from there, Aqua returns to help fight the Unversed.
After dealing with the Unversed, Aqua and Mickey introduce themselves to each other. They figure that the Unversed attacked Kairi due to the light in her heart. Suddenly, Mickey is transported away, but he tells Aqua that he will be fine. Aqua casts a spell on Kairi that will protect her when she's in trouble by connecting her light to that of another. Kairi's grandmother comes to get her, and she tells her a story about how the worlds were created: long ago, the world was one, covered in light. As people began to covet the light, darkness grew in their hearts. They fought over the light, which destroyed the world, but the light inside the hearts of children restored the world in a fragmented state, resulting in the creation of many worlds instead of one large one.
After Kairi and her grandmother leave, Aqua spots a large Unversed. She chases it until she meets up with Terra and Ventus, who were also chasing Unversed. The three Unversed come together to form the Trinity Armor. Together, the three friends manage to destroy it. Ven gives her and Terra passes to Disney Town. Terra mentions a masked boy named Vanitas before urging Ven to go home. Aqua calls Terra out for the bad things he did, and he accuses her of spying on him. Ven angrily storms off, leaving Aqua alone.
Making her way back to town, Aqua meets the wizard Merlin, who shows her the Winnie the Pooh book. Back in the town square, Aqua meets Vanitas, who attacks her. Aqua defeats him, but he escapes into a corridor of darkness, but not before mentioning that he plans to keep her as a "backup." Ven approaches and tells her that he found Terra, but he left. Aqua demands Ven go home before leaving, hoping to find Terra again soon...
