"As the title suggests, the game is set in Ancient Persia. One day, the Sultan of Persia went to wage war in a foreign land, and his vizier, Jaffar, is left to rule in his stead. Jaffar locks the nameless protagonist up because the Princess, Jaffar's love interest, has taken an interest in him. Jaffar then locks up the Princess herself, giving her an ultimatum: marry Jaffar, or die within the hour.
The player character is a nameless protagonist from a foreign country who traveled to Persia. The protagonist falls in love with the Princess of Persia. The primary antagonist of the game is Jaffar, the Vizier of Persia, who imprisons the Princess and attempts to conquer the land while her father is away.
The nameless protagonist escapes his prison, and climbs to the top of the tower, to where the Princess is imprisoned, facing a variety of resistance along the way. The Princess' room is guarded by Jaffar, whom the Prince defeats, saving Persia and rescuing the Princess."
Source: Wikipedia, "Prince_of_Persia_(1989_video_game)," available under the CC-BY-SA License.