A not-very-good Japanese exclusive spinoff of the Phantasy Star series, Gaiden is essentially a side-story following the adventures of Alis Landale's daughter on the world of Copto as an evil force threatens it with destruction.
This game was not developed in-house by Sega, but rather loaned out to a company named "Biox" that produced several other ports and handheld spinoffs, probably the best known of which is Mega Man II on the Game Boy. Whether Gaiden is part of the official series canon is still debated, though Phantasy Star IV's original subtitle was "The Return of Alis", hinting that, if nothing else, it was planned to be part of the series at one point.
Played with a money cheat to cut down on excessive grinding. Audio was muted by Youtube over a copyright claim.
Plot Digest (Plot scenes only, everything else trimmed out)