As a bit of a time reference, Kingdom Hearts I takes place approximately a decade after the events of Birth by Sleep.
The Destiny Islands, a verdant tropical paradise sporting a small village and lots of fun. A trio of friends, the heroes of this series, reside there, knowing not what their fate is.
Sora, Riku, and Kairi have, for some time now, been wondering about worlds beyond their own. They decide to build a raft to travel to them. As they finish it, that night, a freak storm hits like no other. For it is a storm caused not by the weather, but by darkness, that mysterious, evil force that wants to engulf all in eternal death.
Sora, not knowing what this storm is, heads to the island where the raft is to find the island infested by mysterious, dark beings, later named the Heartless, for they lack no hearts; no souls; killing others to transform them into Heartless and to take their hearts to empower themselves.
It is also there Sora finds Riku. As Riku and Sora are surrounded by the darkness, their hands meet, and a bright light shines. The Keyblade, one of only a very few weapons that can combat the darkness, has mysteriously come into Sora’s hands, thereby choosing him.
Riku has also disappeared. Thinking Kairi is still on the island, Sora finds her to no avail, as she is blown away by a dark wind, as is he…
He awakens in an unknown town of another world. There, he meets new friends. It is apparent that this Traverse Town is a place of residence for those who have their worlds taken by the darkness. Sora also meets Donald and Goofy there and they embark on a journey.
Their goals? Sora is out to find his friends again. Donald and Goofy want to find their lost King Mickey. But it is agreed that they must also seal the Keyholes throughout the worlds to stop the spreading of darkness.
As they travel, they learn that Riku has turned to the side of darkness and taken Kairi captive. Eventually, they learn that it is Ansem, a currently mysterious character, who has taken his very body and soul hostage. It all ends in an epic battle versus Ansem.
Kingdom Hearts is found. Thought by many to be a world of darkness, it is actually a world of light, and the light slaughters the Heartless known as Ansem. King Mickey and Riku are found beyond the door to Kingdom Hearts when it opens and, together, King Mickey and Sora use their Keyblades to seal the door.
In an unknown field, we watch the game end as the trio find Pluto, the lost loyal dog of the King, and chase him down…