Fire Emblem Gaiden (ファイアーエムブレム外伝, Faiā Emuburemu Gaiden?, literally "Fire Emblem Side-Story") is the second installment in the Fire Emblem Tactical role-playing game series developed by Intelligent Systems and published by Nintendo. The game was released on March 14, 1992 in Japan. It is also the second Fire Emblem game to be released exclusively on the Family Computer (Nintendo Entertainment System). It serves as a side story to the original game ("Gaiden" is literally Japanese for "side story"). Set in the same game world, the events of Fire Emblem Gaiden actually take place on a different continent, and thus have no bearing on the first game's narrative. However, several characters from the original game, such as the pegasus knight sisters, do make return appearances.
The game follows two protagonists, life-long friends Alm and Celica, over the course of five chapters and takes place in two kingdoms on the continent of Valencia: Rigel and Sofia. The world map is slowly revealed for players to traverse through the game's five chapters. Over the course of the game, it is revealed to both Alm and the player that he is actually the prince of Rigel, whereas Celica is both the princess of Sofia and a priestess of the goddess Mila. Fire Emblem Gaiden was the first game in the series where more than one main character could promote. Alm begins the game as a Fighter before eventually promoting to Hero. Celica is a Priestess, and she promotes into Princess.
Source: Wikipedia, "Fire Emblem Gaiden", available under the CC-BY-SA License.