Kingdom Hearts begins with Sora in a surreal world in which Sora hears many allusions to opening a door, in regards to his fate. He is introduced to odd creatures, and has to fight through many of them before he meets Darkside. He fights valiantly, but Darkside traps him into a pool of darkness, of which Sora is enveloped in. The final words that appear are "You are the one who will open the door."
Sora wakes up to find out it was a dream on the shore of Destiny Islands, his home world. We are introduced to Kairi, who is content with her life yet doesn't remember her homeworld, as she explains to Sora. Riku appears and reminds them that the raft still needs work. It's revealed that they don't know of other worlds outside of their islands and wish to embark on a quest to leave and explore other worlds. Riku seems especially interested.
The next day Sora comes across a cloaked figure in the "Secret Place" (A cave in the middle of the children's Island). The figure comments on Sora's ignorance of the worlds outside of his own and how this world is doomed to destruction, and Sora retorts that he will learn. The figure leaves on this quote; "A meaningless effort... one who knows nothing can understand nothing."
While Sora is in bed that night he notices a storm over the island from his home. He embarks on his canoe to protect the raft from damage but is attacked by the shadow creatures he met in his dream. Finding he can't damage them, he flees to find Riku. Riku comments that the "door has opened" to other worlds. Riku is enveloped by darkness, but Sora does not follow.
Confused and alone, Sora runs to the Secret Place for answers. He finds Kairi, but soon a gust of wind sweeps Kairi towards Sora and, as though she's transparent, goes right through him. He's then he blasted out of the cave.
Darkside appears from Sora's dream and Sora once again fights the monster. Once again, in correspondence to his dream, he is enveloped by darkness once again.