The book presents Mary (the mother of Jesus) at the end of her life, reflecting both on the tumultuous events which took place around her son's death, and her own subsequent life in hiding. She's doubly hunted: by the agents of the Roman Empire, who would like to see her silenced; and by Jesus' followers, two of whom act as her protectors and who feed and house her, but in return for a constant stream of reflections and questions about her experiences of her son. The name of Jesus never occurs in the book - at one point Mary says that she can't bring herself to speak his name, so great is her grief - so it's always "my son" or "him". But he's present throughout the book, his words and his deeds.
Tradition suggests that Mary fled to Ephesus, on the coast of Asia Minor (now part of Turkey) where she lived out the rest of her life, and Colm Tóibín follows that tradition; thus one of her protectors may be taken to be John, the 'beloved disciple', whose oral tradition formed the basis of the Gospel of John. Given my comment above about Mary being hunted, it's striking that Ephesus was the site of the major temple to Artemis, the hunting goddess, which Mary is described in the book as visiting.
Mary's recollections of a number of the key events in Jesus' life that are central to John's gospel - the wedding at Cana, the death and rebirth of Lazarus, the crucifixion - are somewhat different both to the story as we have it in the gospels, and to the story as the disciples are gradually reconstructing it from Mary's accounts. It's very clear that they want to create a story about Jesus as someone supernatural, as the Son of God, and that they see his death as something very distinctive and important.
It's true that the process of creating any sort of biography, even by modern historical standards, is one of selection of which facts to present and how to present them; and this was even more the case in the ancient world. Nonetheless, the way in which the disciples are shown as shaping the narrative around Jesus is striking. It's doing a certain amount of damage to Mary's perception of herself and her life with her son - using both of these to create something new.
They're doing this for enlightened reasons, to reshape the world's understanding of itself, and in the process to bring hope to the oppressed within the Roman Empire. And narratives are inexorably tied up with power - as David Chapman and I have written, "narratives of information are constructed by those in power, sometimes in ignorance of the less powerful and sometimes deliberately to exclude those with less power". Nonetheless, the book ends sadly, with Mary left alone and with her self-perception of her life considerably harmed by those who have supposedly written about her.
The novel reminded me at times of Philip Pullman's book The Good Man Jesus and the Scoundrel Christ, which also shows the developing narrative around Jesus. While Pullman is an avowed atheist, it's tempting to say that Tóibín is presenting a secularised version of Mary's story, but that would be unfair. There's a constant sense of Mary's spirituality - she regularly prays, goes to the temple (Jewish and that of Artemis), and is shown having multiple spiritual experiences. Her version of the narrative - and of course she too is constructing a narrative - is at odds with the conclusions of John's version, but it is no less religious.
Ultimately, all experiences are shaped by narrative, and all narratives are derived from experiences. The only question is what we seek to do with those narratives, and how close we want them to be to the literal truth of our experiences.