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Summary of Trailer:
Video begins by showing scenes of the ending of "Kingdom Hearts 1", Soon the text "Deep Past" comes up, to be followed by "Shut-Eyes gives birth to the concealed fate"

Scene cuts to an area featuring the boy who looks like Roxas, Ven, talking to a young male with black hair, the area seeming to be like "Hollow Bastion". The boy soon calls the young male "Ven." The scene continuously changes before showing the Old Man that is seen in the Kingdom Hearts 2: Final Mix+ Secret Ending.

The old man's name is "Master Xehanort" and "Master Xehanort" calls the young boy "Terra". Soon Terra summons the keyblade into his hand.

Soon some worlds are shown, such as "Olympus Coliseum" and a new world, which resembles that of Sleeping Beauty is shown, confirming it's existence.

The scene then changes to Terra fighting against the mysterious dark male, who looks like he's in a bicycle suit, also from the secret ending. Terra manages to knock off the helmet, but his face isn't shown.

The scene soon moves to King Mickey summoning his Keyblade. Soon the scene begins to change rapidly, and Terra is at a beach, watching two boys playing before resting his sword into the grove. The two boys soon begin to become more visible, and it looks like Sora and Riku playing at the beach.

Finally, Terra says..
"Please don't depend on me, just erase me...friend"

Who are Terra, Ven, and Aqua

An unlockable teaser at the end of Kingdom Hearts II features three unique characters in armor who hold Keyblades. The three characters were in armor primarily to hide their faces until the character designs were completed. Nomura chose armor because he felt he couldn't use the dark hoods like the characters in the unlockable trailer in the first Kingdom Hearts. Nomura has stated that the characters are related to one or more characters already introduced in the Kingdom Hearts series. A new trailer in Kingdom Hearts II Final Mix further shows them with a new optional boss in the game, known as "Todomarishi Shinen" (留まりし思念, "Todomarishi Shinen"? lit. "Lingering Sentiment").

This boss was later confirmed to be Terra and the other two to be Ven and Aqua. Nomura has stated the three's names represent "earth", "wind", and "water", similar to how Riku, Sora and Kairi's names can be seen as "land", "sky", and "sea". Terra is reached through a portal in Disney Castle, in the Hall of the Cornerstone. When Sora meets Terra in a desert, the soldier calls out Xehanort's name and attacks Sora. If Sora comes back to challenge Terra again, he states he has nothing else to give him, and that the only thing he has left is his hatred towards "Xehanort". Their story will be further explored in the upcoming PlayStation Portable game, Kingdom Hearts Birth by Sleep, in the Japanese version of which it has only been confirmed that Ven will be voiced by Koki Uchiyama, who also voiced Roxas.
(source wiki)

Who is Master Xehanort

Master Xehanort (マスター・ゼアノート, Masutā Zeanōto?) is a mysterious old "Keyblade Master" that appeared in a teaser present in Kingdom Hearts II Final Mix. He wears an outfit very similar to the one worn by Xehanort's Heartless in the first Kingdom Hearts. Currently it is known that he and his apprentice disappeared, an omen of a great disaster, and that Terra, Aqua, and Ven are sent on a quest to find him.

Trailer Summary:
First thing that was shown, was the ending of Kingdom Hearts II, afterwards the text "The secret ending is solved, and the worlds still extend" Is shown, and then the words "Flow" is shown.

The video then officially begins with Jiminy Cricket looking at his journal, the words "Kingdom Hearts: Chain of Memories" pop-up. He then notices something he did not right in his journal "You need to return, to heal the wounds" Jiminy Cricket then rushes to report to King Mickey. Mickey who is startled by this only replies with "Only he is the one who can solve it." The scene soon changes to Sora standing in Destiny Islands, in Kingdom Hearts 1. However Sora only appears in the form of computer data that is being transmitted.

After the image, the game soon changes to a battle scene. Heartless are attacking, and the gameplay mechanic seems odd, as Sora appears to be moving pies of blocks making it a puzzle game in a sense. Soon the command "Debugging" comes up. When this command is clicked, Sora moves at high speed, and vanishes as well. The scene soon changes to Sora being in Traverse Town with Mickey (As seen in the latest scans) With Mickey saying "I was here on that day." and then "Let's speak truly"

Plot
Coded is set after the events of Kingdom Hearts II.Jiminy Cricket returns to Disney Castle to organize the records in his journal. In his journal he finds a message that he did not write,"We must return to free them from their torment." To investigate this message, King Mickey digitizes the contents of the journal, awakening a virtual Sora on Destiny Islands. (source wiki)