The plot evolves around a boy who is severely ill and can't get out of his bed. Their parents try everything so as to get the boy to wake up from this sort of 'coma'. It turns out that it is no ordinary disease, in fact, the invalid is actually trapped outside of its body because it is currently being occupied by another spirit. The father after being put into trance travels to this parallel world in order to bring his son back from this trauma and until the very end you are convinced he was able to do exactly that. In the end, the family takes a photograph as they are united again. In the photograph the aura of an old woman is visible thus representing that the soul that possessed the son is now in hold of the parent. The photographer after realizing what had occurred is chocked to death by the spirit which then turns around to where the remaining of his family went.
That would be Insidious, from Wikipedia: