I bought this game off of the PSN Store last year along with a physical copy of The Phantom Pain. Ground Zeroes takes place a year after Peace Walker with Big Boss going to Camp Omega in Cuba, with the mission of saving Paz and Chico who were imprisoned there. After beating the main mission... well, that's it really. You get the ending followed by some unlockable side ops depending on what rank you achieve. For a prologue, it does a pretty good job at building up the story into The Phantom Pain
There are a few other side ops to do in this game, but that's really all I have to say on that matter. I mean, the gameplay is awesome and it visually looks amazing for the PS3, but there isn't much else to say on this game. I didn't feel the need to go back and unlock the rest of the side ops as the main mission was done. In my honest opinion, Ground Zeroes may be my least favourite Metal Gear game in the franchise due to the shortness and lack of replayability after that one main mission. This is like if they took MGS2's Tanker Episode and released it as a separate title with a few side ops/Snake Tales. Even other games like New Super Mario Bros 2 or even Gravity Rush was longer than this. Luckily, I didn't buy it for whatever it was priced on release. It's not a bad game by any means, but I don't see much reason to come back to this game.
At least it did introduce one of the dankest memes in a video game ever used...
