From the design document:
One hundred years have past [sic] since the Alien Invasion of Corridor 7. An evil
intelligence has once again assembled a deadly army of warriors, bent on conquering the universe. Total destruction of the
planets Ailoprobe and Semaj by the evil force has convinced other alien races to fight against them. This has created a
chain reaction of events, resulting in the unification of the Rodex, Mechanoid and Human race in an attempt to fight the
Corridor 8: Galactic Wars takes you there! Which side will you choose?
Corridor 8: Galactic Wars was to be the sequel to Corridor 7, but never made it past a prototype stage. There aren't any screenshots here because there's nothing really to take a screenshot of, especially since many Doom graphics are used as scratch art. The game was somewhat ambitious according to its design document, with six episodes of ten maps each, divided into two campaigns of three episodes: an 'allied' and an 'axis'.
The source code and prototype of this game, plus the design documents, can be found at KBW's Corridor 7 Page.