Storyline of Nell's 그리고 남겨진 것들 (The Day Before) MV
Basically, Lee Minki (the one with the book) has committed suicide by taking poison and has (most probably) filmed it. His friend (portrayed by Song Jaerim), relives it and other conversations they had while viewing it.
The book Lee Minki has in hand is John Berger’s ‘And Our Faces, My Heart, Brief as Photos’.
The book is a key to this puzzle - John Berger writes about the images of the dead living on into present thought. The book is in 2 parts: Time & Space. Minki reads the page before and the poem “A view of the Delft” before he takes the poison. The lyric “I wonder if we are sometimes standing in the same memory, I wonder if sometimes, time has stopped for you too” are also clues. Another quote from the overview: “Those who read or listen to our stories see everything as though through a lens. This lens is the secret of narration, and it is ground anew in every story, ground between the temporal and the timeless… In our brief mortal lives, we are grinders of these lenses”.
(This is my view from what I could infer and from the information I gathered)