Metal Slug was one of the surprising series to come out on the Neo Geo. It came out long after most people had totally forgotten about the arcade hardware platform, which was (and still is) used mostly for fighting games. The Metal Slug games were linear, side-scrolling Contra-like shooters, pitting one or two players against an enormous army. The Neo Geo Pocket Color Metal Slug game successfully duplicates the look and gameplay of the arcade games, but it also adds a non-linear element to it, which brings a whole lot of depth to the series.
There are lots of branching pathways in the game. Some are as simple as walking out a door, while others involve cool little things like getting shot down while piloting a jet, and having to eject and parachute down to a level you wouldn't see if you were better at dodging enemy aircraft. Also, you've been given a health meter, so you can withstand a few shots before going down. Your maximum health increases as you progress through the game. All of the little nuances of the arcade games are present here, from weapons like the shotgun and missile launcher, to vehicles like a jet and the Metal Slug itself. You can knife soliders, collect various power-ups, rescue POWs, and even grab coins, which can be spent on continues. You can also save your game.
The game has really good graphics for a portable system, and most of the game runs at a pretty high speed. However, the game tends to slowdown when there's a lot of action on-screen. Thankfully, it isn't too bothersome. Metal Slug: First Mission is a must-own game for shooter fans. It has the longevity to survive long after the game has been beaten.