Metal Slug takes the tried and true "one or two men against an army" storyline that has been a part of video games since the 80s and adds in rescuing POWs. In later games in the series, the enemy soldiers are replaced with aliens towards the end of the game and all hell breaks loose. Back to those POWs for a second: they may be some of the most entertaining NPCs seen in a video game in the 90s, as well as some of the most useful. They give you weapons, point bonuses, and on rare occasions, they will fight alongside you. I remember the first time I rescued a POW and he not only gave me a homing missile power up then followed me for a little while throwing fireballs at enemies like he was Ryu from Street Fighter.
I have gone this entire post and haven't even touched on the vehicle that the series is named for. The Metal Slug Tank is a throwback to S.O.P.H.I.A. from the Blaster Master game, a monster that will maul anything in it's path. Maybe I should say it will as long as it doesn't take too much damage. So maybe it's less like S.O.P.H.I.A. and more like that thing at the beginning of Contra III. While Metal Gears and Rush have changed, the Metal Slug has remained the same over the course of the series, much like a classic Pickup Truck. If it ain't broke, no need to fix it though, is there?
The Metal Slug series is 15 years old this year, but it's heritage runs deeper. It's gameplay is steeped in 8 Bit and Arcade tradition, and development team at Nazca, and publishers at SNK poured every bit of their history into what became the last great 2D action game series. If you still own a console that will play it, I advise you to track down a game in the Metal Slug series. With the exception of the first game on the Neo Geo AES(the MVS version is much cheaper), you can find may games in the series for decent prices. I am always willing to recommend something good to fellow gamers and I fully recommend the Metal Slug series to anyone looking for hardcore side scrolling action.