I've spent quite a lot of time discussing "The Seven Basic Plots" by Christopher Booker, and having covered a lot of ground, I want to comment on the conclusion he draws. First, it's worth following the structure of his argument. This won't do it justice, I'm afraid - his book covers over 700 pages! But hopefully it will at least be accurate as far as it goes.
He starts off by demonstrating that many of the narratives which we are familiar with fit into a small number of standard plots - namely: Tragedy, Comedy, Overcoming the Monster, Quest, Rags to Riches, Voyage and Return, and Rebirth. He then shows that these are actually all really one fundamental story - which involves the central character of the narrative starting in a form of darkness or incompleteness, and either remaining there and facing destruction (in the case of Tragedy, and some Voyage and Return stories) or moving into light and completeness. Completeness is archetypally expressed through the union of male and female ("they married and lived happily ever after"), and will often have wider implications than simply those for the central character ("there was joy throughout the kingdom"). This, he argues, is because narrative is designed to show us the triumph of the Self - that is, humans realising their full potential as physical, mental, emotional and spiritual beings, expressed as the union of masculine and feminine characters - and that this triumph of the Self in a person is something which gives benefit to humanity more widely. Where the Self doesn't triumph - where a person is not able to fully realise themselves in this fourfold way, due generally to their ego preventing it - it leads to unfulfilment, despair and ultimately destruction, and humanity is the worse for it.
He adds a couple of more modern plot structures to this ("Mystery", "Rebellion against the One") and comments on them, and also talks at length about how the rise of the ego following the Romantic era has led to a breakdown in narrative structure. Rather than the satisfactory resolutions we see in the seven basic plots, these narratives with their focus on the gratification of the ego can be reduced to a kind of unsatisfactory fantasy. Describing large swathes of post-romantic literature and narrative as having unsatisfactory plots may come across as reactionary - however, the case is made at length, and certainly whilst (say) "Basic Instinct" and "Close Encounters of the Third Kind" make gripping movies, you are left at the end of them wondering "What now?" This is a very different response from our response to a conventional narrative.
He then traces the idea of the ego and the Self through history. He takes as his starting point the idea that the separation of the ego - our own personal desires and aims - from the Self - the interests of the group or species more widely - is the thing that separates us from the animal kingdom. Narrative is built into us, as a mental pattern which encourages us to act in a Self-oriented way, rather than an ego-oriented way. This has expressed itself in many ways through history, including religion, but following the rise of romanticism, the industrial era and so on, there has been an increasing move away from the Self and towards the ego, with the consequence of a breakdown in "Self" aspects of society - stable societal structures, altruistic behaviour, neighbourliness and so on. The historical analysis is effective - for example, he points to the way in which the fragmenting trends of the 1920's and 30's in the UK were reversed in the Second World War and its immediate aftermath, as people behaved in a "Self"-oriented way to challenge the threat they faced from a shared enemy - only for the breakdown to continue where it had left off from the mid 50's onwards.
In conclusion, Booker argues that as we understand this as being the proper role of narrative, we can see its place in human society. Also in contrast to the indifference to behaviour which characterises a post-modern, pluralistic society (he doesn't use those phrases!), this ideal of Self suggests that there is an absolute standard for behaviour which is good for humanity - that there is a sense in which gratification of the ego, or an unbalanced pursuit of physical, mental, emotional or spiritual fulfilment in a person is harmful, not only for them but for humanity more widely.