Super Robot Wars Alpha Gaiden (スーパーロボット大戦α外伝 Sūpā Robotto Taisen Arufa Gaiden), for the PlayStation, is the first side-story in the Super Robot Wars Alpha series, continuing from Super Robot Wars Alpha. Super Robot Wars Alpha Gaiden, or simply, Alpha Gaiden, includes several rare series choices, such as After War Gundam X, Turn-A Gundam and Xabungle, all of which debut in the game.
Shortly after the events of Alpha, the Earth Federation has covered up many of the events in the climatic battle with the antagonist Aerogaters, but the gravity shock wave from the detonation of the Excelion (from Gunbuster) in the Battle of the Solar System's Absolute Defense is rapidly approaching the Earth, and threatens to wipe out the space colonies and the planet. The Earth's only hope is the Aegis System, which could shield the Earth Sphere from the shock wave.
Unfortunately, the heroes are sent to an alternate future, where the Earth has been ravaged by the wave. Faced with a new threat in the form of the Ancestors, the heroes must find a way to return to their time and prevent this alternate, dystopic future from happening.
Super Robot Wars Alpha Gaiden also includes the final appearance of the Masō Kishin "Banpresto Originals" in the Alpha series, and the last appearance of the characters, besides Masaki Andoh and the Cybuster, Shu Shirakawa and the Granzon, and Lune Zoldark and the Valsione. Although many fans suspect this is due to a copyright backlash with Winkysoft, it is stated in an interview with the creator of Super Robot Wars he merely wants to give more plot attention towards the other original characters, considering the previous titles have dealt with the Masō Kishin plots rather extensively. Additionally, the game features the first appearance of Sanger Zonvolt, who later becomes one of the selectable protagonists of the 2nd Super Robot Wars Alpha.
Gameplay wise, Alpha Gaiden is the first Super Robot Wars game where all the weapons of a unit are upgraded simultaneously, this would later go on to be used in almost every game made after Alpha Gaiden, save for the Original Generation games. Alpha Gaiden also introduces the Support ability for Attack and Defense, allowing units situated beside one another to attack after each other, or take an oncoming attacking. This ability would also go on to be used in almost every subsequent game. In 2009, Super Robot Wars Alpha Gaiden was also fan translated almost entirely to English.
Source: Wikipedia, "Super Robot Wars Alpha Gaiden", available under the CC-BY-SA License.