The games focus on the Peregrine Falcon Squad, a small group of soldiers who fight against a rebel army, aliens, zombies, mummies and various other forces intent on world domination.

Over the years since its debut, the franchise developed a small, but passionate cult following due to its unique visuals and slapstick humor.

The run and gun elements pit a player against many enemies with the aid of extremely powerful weapon pick-ups.

The vehicle can run over infantry and perform a suicide attack, with the driver jumping clear and the tank ramming into and exploding upon the target.

Some vehicles will only provide Vulcan cannons (which are more powerful than the handgun's bullets and are also unlimited), but neither special bombs (grenades must be used) nor armored protection to the player.

The first game's story involved the Peregrine Falcon (PF) Squad, a small but skilled team of soldiers serving under the Regular Army's special operations division, who fight against the army of General Donald Morden in order to prevent a massive coup d'état and the creation of a New World Order.

The fifth moved to the motif of modern guerrilla warfare, leaving only traces of the series' signature quirky humor and paranormal enemies, with the exception of the final boss.

Metal Slug 6 returned to the plot of the first three installments, bringing back Morden's Rebel Army and the Mars People.

Metal Slug 7 has less outlandish elements with the Mars People replaced by an alternative universe of Morden's Army with futuristic equipment and weapons.

Major Marco Rossi (voiced by Takenosuke Nishikawa in NeoGeo Battle Coliseum[3] and by Kenta Miyake in Neo Geo Heroes: Ultimate Shooting) and Captain Tarma Roving were the only playable characters in the first game, and each was reserved solely to the first and second player, respectively.

In the fourth game, Nadia Cassel and Trevor Spacey made their debut, replacing Eri and Tarma.

They have not returned in later games, as they were created by the Korean-based Mega Enterprise and due to Playmore retaining intellectual rights to all SNK titles.

Despite the fact that Allen is clearly killed at the conclusion of every encounter, the game makers have humorously brought him back in each new iteration of the series.

They resemble squids, using their tentacles as a form of movement, and a strange fighting style that involves gas and a laser pistol.

The aspect of the Metal Slug series was to create a simple, but exciting side-scrolling shoot-em-up game with an easy control scheme (one joystick and three buttons).