One year after the events of Metal Slug 4, a special disc that contains deep and intricate secrets about the Metal Slug project is stolen by a mysterious group called the Ptolemaic Army, whose specialty lies from within archaeological excavation and espionage. Marco and Tarma of the Peregrine Falcon Strike Force follow in hot pursuit against the group and in the process are joined by Eri and Fio of SPARROWS. Together once more, the quartet investigate the shrouded objective of the Ptolemaic Army, who over time grows more powerful as they are joined by a mysterious masked man and his followers. At the end of the game, the Ptolemaic Army summons a giant demon as the final boss, which after a long battle is forced to leave Earth thanks to the heroes.
Metal Slug 5 received mixed to positive reviews from critics.[5][6][7] It received very positive scores from players, with an average of 8.1 on GameSpot for the PS2 version and 8.4 for the Arcade version.