Kingdom Hearts Birth by Sleep is set ten years before the first Kingdom Hearts game.
Birth by Sleep is a prequel that explores the earlier era of the Keyblade legacy, long before Sora's original adventure. Its story follows three playable heroes—Terra, Ventus, and Aqua—as they train under Master Eraqus and become involved in the search for the missing Master Xehanort.
The ten-year gap gives the game room to explain the origins of characters, rivalries, and conflicts that shape the original Kingdom Hearts. Several settings and relationships are shown at earlier stages, while the events establish important background for the series' later timeline.
The structure can make the chronology seem more complicated than it is. Players experience separate character scenarios rather than one continuous campaign, and each hero views familiar worlds from a different perspective. Birth by Sleep originally launched for the PlayStation Portable in 2010; its Final Mix version and later HD releases do not change its position as a prequel set a decade before the first game.