The basic idea is that there is this group that, over the centuries, has learned to control reincarnation. John's character stumbles into that realization, and it's a lot closer to him than he would ever have wanted to know. It's a metaphor for when you get in a fight with your significant other and you go, "Who is this?," or you look in the mirror and go, "Why did I say that?" It's the intruder. When you threw a temper tantrum at two years old, it's them.
