"Metal Slug 2 (メタルスラッグ 2) is a run and gun video game for the Neo-Geo console/arcade platform created by SNK. It was released in 1998 for the MVS arcade platform and is the sequel to the popular Metal Slug. It is the second title in the Metal Slug series. The game was later re-released in a slightly modified/remixed form as Metal Slug X. It was re-released for the Wii's Virtual Console in North America on December 1, 2008 at a cost of 900 Wii Points.
General Morden, the antagonist from the first game is back once more with his army, bent on taking over the world. It is up to the Peregrine Falcon squad, who are now joined by two new characters: Eri Kasamoto and Fiolina Germi of the Sparrows Intelligence Unit, to save the day.
As the levels unfold, it turns out that Morden has allied with Martians to help facilitate his plans. In the final level of the game, the tables are turned when Morden comes under attack and is betrayed by his Martian allies and taken prisoner by them. An ad-hoc alliance is formed between the Peregrine Falcon squad and General Morden's army to combat the greater alien threat. After a long battle, they succeed in defeating the Martians' Mothership, driving them off the Earth. Morden seems to have fallen from the ship, strapped to a solid iron plate. While his soldiers celebrate, the plate loses its balance and crushes him.
A revised version of Metal Slug 2, titled Metal Slug X, was released on March 1999 for the Neo Geo MVS. It fixed problems with slowdown present in Metal Slug 2, and increased the difficulty. Metal Slug X also introduced a few new elements to the Metal Slug 2 game system. New weapons and items were added, such as the "Iron Lizard" and the "Drop Shot". The enemy placement and bosses were re-arranged as well."
Source: Wikipedia, Metal Slug 2 , available under the CC-BY-SA License.