This entry in the Metal Slug series features the theft of information on the next generation of Metal Slug by the "Ptolemaic Army", wherein our heroes are tasked with recovering that data and putting an end to the evil group.
The author improves his previous version by using a new strategy on the last boss. You can find out the details in the submission text.
You can also see more of x2poet's Metal Slug work by looking at his runs from Metal Slug, Metal Slug X, Metal Slug 3, Metal Slug 4, and Metal Slug Advance.