[Big Boss is back in a brand new adventure]
The game hasn't been given a year it takes place, but from the trailer/gameplay I will post below; it is a sequel set at least two years after Metal Gear Solid: Peace Walker
When you watch it click the spoiler tag below
If you are wondering who the boy is in the cage this is Chico as it was mentioned in the translation at the end of the extended cut of the clip which featured a tiny bit more gameplay. It also makes sense as he betrays a woman, which in this case will be his big sister Amanda. Paz the girl from Peace Walker is also mentioned being tortured as well, they are scared she will reveal information on this new band XOF. Big Boss is in his late 40's early 50's in this game and as we can see is apart of FOX now as well. In terms of plot it seems to be the group XOF (FOX backwards) are two armies that formed from Big Bosses 'Mother Base' as later in the video the new 'burned man' mentions 'We are an army without borders' which was the concept behind Big Bosses Mother Base.
There is no release dates for the game but we should hopefully expect a bit more news at the Tokyo Game Show towards the end of the month. Here is the video to the normal cut of the gameplay trailer.