“The Kitakousai festival was coming up at school, but the members of SOS were running around making the movie out of Haruhi’s idea. What happened at this busy time of the members? Taking place in the back stage that wasn’t shown in the anime, players are able to enjoy the original story of the game. The secret that leads to the TV anime is now revealed!!”

The PSP game is an interactive novel with spot animations and voice acting that's provided by the actors from the animated series. Players take on the role of Kyon and engage in various interactions with the main cast members and read the conversations that ensue. Although the story in Suzumiya Haruhi no Yakusoku is based on the "The Sighs of Suzumiya Haruhi," the scenarios and overall material are new and written exclusively for the game. Multiple endings are also possible depending on the actions that one takes over the course of the game. In addition to the interactive experience, a number of mini-games are also playable at certain junctures including a simple beach volleyball diversion. Bandai touts a new "Motion Portrait" technology that's used in the game where lifelike facial expressions are imbued on the characters so that they appear to be "living" renditions of the heroines we've grown to adore.

[Synopsis from TUNAFISHISGOOD]