The Metal Gear series has a complex and convoluted plot. Generally, it involves a lot of military and political intrigue, with the main character often facing off against powerful military organizations. For example, in many of the games, Solid Snake has to infiltrate various bases and deal with threats like the Metal Gears, which are powerful bipedal war machines.
Metal Gear Solid 5 marks a significant conclusion. It wraps up key elements and provides a sense of finality. Though the possibility of additional stories in the same universe can't be ruled out completely.
The story of Metal Gear Solid 5 is decent. It has some standout moments and themes, but it may not be everyone's cup of tea. Some players might have expected more from it.