A fun retro run and gun arcade game. One year after the events of Metal Slug 3, the world is trembling under the new threat of a mysterious but deadly cyber virus that threatens to attack and destroy any nation’s military computer system. With Tarma and Eri unable to help due to their own assignments in the matter, Marco and Fio are called in to investigate the situation and are joined by two newcomers, Nadia and Trevor. In their investigation, the group discovers that a terrorist organization known as Amadeus is behind the nefarious plot and that they head into battle against Amadeus’ forces, hoping to destroy the cyber virus before it gets the chance to wipe out the entire world’s military computer system. Halfway through the game’s story mode, the player is confronted by one who they presume to be Morden, however in the final stage they find an underground facility where Terminator-like doubles of Morden are being manufactured. Allen O’ Neil (who has appeared numerous times throughout the series) fights the player in this stage for the very last time, and is also revealed to be a machine like the Morden doubles, before he is destroyed and the player confronts the leader of Amadeus,the businessman from Metal Slug series Amadeus himself.