Birth by Sleep is probably one of my favorite Kingdom Hearts Games. Click here to find out why.
This game has the best combat of the series, with abilities like the new Command Deck (Also in Recoded or Dream Drop Distance) and D-Link. It's also great that the three characters specialize in some ability (Like Ventus having fast attacks, Terra having strong attack power, and Aqua being great at Magic)
Anyway, the story.
Taking place ten years before the events of Kingdom Hearts 1, this story begins in the Land of Departure, where three Keyblade wielders and close friendsTerra, Aqua, and Ventus (more commonly known as Ven)-train to become Keyblade Masters under Master Eraqus. To achieve this rank, Keyblade Apprentices must take the Mark of Mastery exam once they come of age. Terra and Aqua take this test together and in the first round Master Xehanort sabotages the exam, forcing Ventus to intervene. In the second round Terra and Aqua fight each other, but while Aqua passes the test, Terra is deemed unfit to become a Keyblade Master due to the abundant darkness in his heart.
I really have nothing to say other about the game then it's plain awesome!