What I got from the many theories which mostly explain the same things but with little difference in details. Here it is, it's kinda long I should warn you: In the music video of "I Need You" we are shown how the boys deal with different hardships in their lives which lead them to suicide or to commit something awful: Suga burns the house down, Jimin drowns himself, J-Hope swallows a bunch of pills, Rap Monster is kinda unknown because they only show us how he drops a cigarette (I don't know), Jungkook is beaten and after hit by a car, V kills a man which I think is his dad, and Jin is the key character because he does not actually do anything. The later story in the Prologue made me think about the scenes with Jin, that he only suffers because of the death of his friends. The "I Need You" music video ends with specific single shots of only V and Jin, which makes me believe that they are the only ones alive.
Then, the Prologue begins with V all bloody after killing his dad, and he calls someone (Hyung who I think is Jin), and tells him that he needs to see him. After that we see V kinda waiting for Jin, but meanwhile the rest of the boys appear, but they don't hug or anything, they just wake him up. When Jin appears V runs and hugs him, action that made the fans believe Jin and V are the only ones alive, and the rest are just fading memories. The five members are shown running and having fun like they are in Heaven or a place between the real life and the afterdeath. Jin is obsessed with recording everything which could be interpreted as him living through the memories which he had with all the boys, when in fact he is there only with V. The paper bag floating around shows that the place is actually empty. In the next scene Rap Monster is writing something (which was translated by someone as "We/You need to survive") that could represent him talking to Jin and telling him to "survive" the loss of his friends (again we don't actually know what happened with Rap Monster - we could say that he is at the limit between life and death as he can communicate both with the boys and Jin).
The next scene shows us the boys around the fire (like they used to in the past) but none of them except Jin actually speak, because the dead can't speak. Suga has a moment with the flame of his lighter making us think about the fire he died in. J-Hope covers a sleepy, cold Jimin who died by drowning. Jin then looks at a photo (image which is shown at the end of the "I Need You" music video) of the sea and asks the boys to go there, they all nod, but don't say a thing. The sea is the place where they were all together and happy, and both Jin and V go there to find some kind of comfort or consolation about the death of their friends. Jin records himself having fun, like he did while the others were still alive.
Then V sees a high platform and decides to climb it. He looks once again at Jin while he is still beside him, and when he reaches the top of the platform, kinda showing remorse. On the ground Suga, Jimin, J-Hope and Jungkook encourage him to jump (and to come to them - they being dead), but Rap Monster is kinda like "What are you doing, man, don't jump!" (which again, leads us to think he is stuck somewhere in between). We never actually see V reaching the water, but we are left with the impression that he drowned, joining the rest of his dead friends.
At the end we see the most mindblowing scene which led to this whole theory: Jin is in his car and from which we can see he is surrounded by water (in the Prologue Rap Monster hits a little car with a golf bat which landed in water-another clue), we don't know if he is in the water or besides it. But then he takes a polaroid photo Rap Monster took back in the gas station with Jin and Suga, but Jin only sees himself in the photo, so where is Suga? It looks like a selfie, because Jin was actually alone and he imagined being with the boys.