A young boy of 14, Sora and his friends Riku and Kairi dream of visiting outside worlds away from their home on Destiny Islands . So they decide to build a ship to sail out of there, but the night before Sora has weird dreams about darkness and light. He wakes up not knowing if their real or not. When awoken Sora sees a whole bunch of heartless on the Islands. He tries to fend them off, but is to late and Sora's world is completly engulfed into darkness. While this is happening, King Mickey Mouse leaves to try and discover on the dark forces that are growing in the universe. He has his friends Donald and Goofy go and find the keyblade wielder, who is chosen to fight off the darkness.
Sora finds himself at a completly different world called Traverse Town where he meets a man named Cid who runs the Accessory Shop. Sora later bumps into a brooding man named Leon, who can't believe that a kid is the chosen one. Leon and his friend Yuffie tell Sora all about the keyblade an how it's supposed to fight off the darkness and save other worlds. Cid, Leon, Yuffie and Aerith all lost their world too. While Leon explains exactly what the keyblade does, Donald and Goofy are looking around Traverse Town for the Keybalde weilder, and to their surprise they happen to fall right on him....literally. When this happens a gigantic heartless appears and Sora, Donald & Goofy fight it off. Sora says that he needs to find his friends Riku and Kiari. Donald had no interest in this, but had their own agenda on finding King Mickey.
Sora, Donald & Goofy bord the Gummi ship which can take them to different worlds. The first world they land in is Wonderland. Sora and the others find themselves to big for the place and drink a shrinking potion to be able to move around. They then find themselves in the middle of an unfair court hearing on a girl named Alice. The jude was a Queen who kept screaming "off with her head!" Sora decides to try and help Alice by finding evidence to prove her innocense. But the Queen of hearts wouldn't listen and locked up Alice and sent cards on Sora. Sora fights off the cards, but when he tries to help Alice he sees that she is gone. Sora then has to fight off a heartless called the Trickmaster. He defeats the heartless and see that Alice is no longer there, so he decided to pack up ship.
On their next stop at Olympus Coliseum Sora, Donald & Goofy all meet a little goat man named Phil. Phil is the trainer of the legendary Hercules and Phil tells Sora that if you wan't to be a true hero you have to train and work hard. Sora then bumps into Hades, who gives Sora an entry pass into the Coliseum. Hades has a couple of other things he want's to enter in the tournament, like Cloud and his hell hound Cerberus. Sora battles Cerberus and beats him. He then talks to Cloud and tells Cloud he will one day find his light and be able to pass through the darkness.
When traveling in the Gummi Ship, they see a jungle looking world. Sora wants to land there just in case if Riku and Kiari are there, but Donald says they should keep going. After much arguing between them they land in the Deep Jungle where they are attacked by a lepord named Sabor. Tarzan the jungle man, saves the gang and leads them to his freind Jane's camp sight. They alos meet a man named Clayton who seems to like hunting things a little too much. Tarzan helps Sora defeat heartless who were there trying to harm the gorillas. They also see that Clayton tried to hunt some gorillas himself. They confront Clayton and fight him and a stealth sneak heartless. After this Sora seals the keyhole and apologizes to Donald about there squable from earlier.
Stoping at Traverse Town to learn more information about the journey, Sora, Donald & Goofy run into Riku who seems to be acting really different. After a short conversation Riku leaves without another word and leaves Sora confused. But Sora now knows at least one of his friends are okay.