X, of course, is not the tenth entry in the series.  Rather, it's a rework of Metal Slug 2 released a year after the original that makes a number of core changes.  Among these are remixed music, altered enemy locations/difficulty, added weaponry, and a new announcer.  This Xbox One version of the game comes to us courtesy of the ACA NEOGEO series, which works to replicate NEOGEO classics for the modern gamer.

Metal Slug X continues where Metal Slug left off, just in case you were following the "story". The bad guy has augmented his soldiers with the help of some aliens, and in order to put a stop to all the evil stuff, the protagonists from the original game pick up guns and start running around shooting everything. After all, that's what Metal Slug is all about. Not aliens and villains, just moderate to heavy amounts of shooting in a detailed pixel world.

Metal Slug X continues the story of where the original Metal Slug left off. The bad guy from MS1 decides to augment his not-quite-Nazis-but-totally-Nazi army with alien forces from beyond the stars. Now, the two guys from the last game have to team up again to-- You know what? Don't worry about it. Run forward, shoot and knife bad guys, and occasionally get turned into mummies. That's all you need to know.