Here is a start on a formalization.
Narratives are first class objects.
Intutively, narratives are collections of events and situtaions and relations among them. Tentatively, we will not use sets in our formalism. There are two ways of looking at narratives that elaborate other narratives, and maybe we need both of them. We may be asserting that N 1 occurred, and N 2 also occurred and gives more detail. On the other hand, we may regard N 2 as a mere hypothetical elaboration of N 1, even a counterfactual elaboration.