As their first step toward the conquest of the Crescent Galaxy, the robotic armies of Mardoul have centered their malevolent mechanical designs on the invasion of the seemingly medieval colony world of Earst. Their expected easy victory against mere knights in shining armor, however, is shattered when Prince Joldy Volder confronts them in a secret weapon that has lain in Earst's hidden underground bunkers for millennia: the legendary giant robot Galient!

[Source: Sentai]

The first two episodes are summaries of the TV series, the third one (Crest of Iron) takes place in an alternate universe.