The original has you fighting your way through Morden's soldiers to the tune ofFinal Attack, eventually falling off a bridge and onto a boat. From there, the boat reaches an island where you have the final showdown with Morden himself in his helicopter.
2/X has you fighting against the Mars People. When you reach the end, the Mars People abduct Morden and you have to fight off the Mars People in their giant mothership. Also, the battle has the Rebel Army aiding you against the mothership, this being the first time in the entire series that the Rebel Army helps you. Also, their shots all count on the mothership. Let's not forget that the battle with the mothership is a Shout Out to Independence Day...
3 is known for having one of the longestand hardest missions in the entire series. You go through a long dogfight with Morden, Allen, and the enemies, eventually fighting Morden himself in a remake of the original's final boss battle... Only to discover that that Morden is actually a Martian in disguise and he abducts your character. You then go to space and storm the Mars People's mothership, freeing Morden and your captured character in the process, and finally ending with a free falling climactic battle with the leader of the Mars People, Rootmars.
4 has you rappelling down Amadeus's base Mission: Impossible style, fighting against enemies and scientists that fire bullets which turn you into a monkey if shot. However, being a monkey gives you unlimited Heavy Machine Gun ammo, and you face Allen O'Neal again except he's a robot. Then you have the final showdown with Amadeus himself, and after defeating Amadeus, you must then escape the exploding base!
6 has you going underground into the Venusians' hive, and to make it better, both Morden's infantry and the Mars People will lend you a hand for this mission. This mission also has a much darker versionof Final Attack, signalling that the Venusians are much more dangerous than Morden or the Mars People. Upon reaching the end, you must battle your Brainwashed and Crazy friend who's under the Venusians' control, and you battle the Venusian Queen herself while driving a golden Metal Slug.
7/XX has you fighting Morden's soldiers from the future and eventually reaching the time portal from which the Future Rebels came from. Upon destroying the portal, a massive machine comes from the portal and Morden gets inside of it. Then, at the end, you battle the Kraken which is controlled by Morden on top of lava. All to the tune of this remix of Final Attack.