You are the one who will open the door.
The flagship game, Kingdom Hearts is what began the series as we know it today. It was first released in Japan on the PS2 in 2002, and has since been remastered (and rebundled) onto the PS3 in 2013 for its tenth anniversary. A joint project between Disney Interactive and Square Enix, Kingdom Hearts combines various Disney worlds (and characters) as well as Square Enix characters, typically from the Final Fantasy series. All the worlds are connected, and its up to the Keyblade wielders to keep them in the Light and prevent them from plunging into Darkness, and the Heartless, by extension.
Sora is the titular character, and the opening game starts with him and his two best friends, Kairi and Riku. Whilst Destiny Islands is the home world for the boys, Riku expresses his desire to leave it, which leads to the three of them building a raft to get out there and see all those other worlds. Kairi's from one of them, so they must be out there, right? After several days of preparations, the three agree to set sail the following day.
However, this plan is quickly turned upside down as that night, Destiny Islands is consumed by the darkness and its light goes out. Riku, enticed by it, disappears, and Sora is the one to take the Keyblade and go in search of Kairi, who likewise, disappears, taking refuge inside of Sora's heart. It's not long before Sora is tossed aside into Traverse Town, along with many other world refugees, and whilst he begins his search for his two friends he runs into Donald Duck and Goofy, who are in hot pursuit of the Keyblade wielder. Thankfully for them, Sora's just the guy they're looking for, and this is when their journey truly begins, setting off to various Disney worlds in order to seal the Keyhole for each and save them from being destroyed as Destiny Islands was. Along the way, they battle many, many Heartless and save just as many people.
In the end, Sora reunites with both Kairi and Riku, but is only fleeting. Kairi is returned to the restored islands, and Riku - along with the King, Mickey - is trapped within the realm of Darkness. Sora and his companions set out to find them...