Essentially, the story follows the actions of protagonist Robert Langdon, a symbologist who studies religious symbols and their meanings. He is framed for the murder of the curator at the Louvre museum in France. This is where many famous works of art, including the "Mona Lise" of Leonardo da Vinci, are on display. The murdered man leaves clues as to who the murderer is. His granddaughter, a very intelligent cryptographer and code breaker, is brought in to assist in solving the mystery.
The story turns into a search for the missing Holy Grail. Supposedly, it has been kept hidden by the Catholic Church, for fear it would overturn all that the Church teaches as true. The secret is supposed to be that instead of having been crucified a bachelor, Jesus Christ had actually married Mary Magdalene. It's also said that he had children, leaving a bloodline that lives on to this day.
It is claimed that the Church has slandered Magdalene as a prostitute, doing everything it can to hide this secret that she was really the wife of Jesus and mother to His children. There is a lot of dialogue regarding this. Also, there is talk of the Gnostic gospels and other literature proposed for the Bible, but rejected by the Church as having been inauthentic. Essentially, it was not written by the authors they are claimed to be written by.
There is also the claim that it wasn't until the 4th century that Jesus was only believed to be divine. This was many centuries after His death, around the time of the rule of Constantine and the Nicene Council. Before then, all His followers believed He was merely a mortal man, and not an immortal incarnation of God. There are also claims by the characters that the Bible had been altered, mistranslated, and ultimately tampered. Over time, this was done to obscure and falsify the "truth" about Jesus, especially about His relationship to Mary Magdalene.
While these ideas are interesting and make for an intriguing story, I find them unconvincing. Having studied the Bible and Church teaching, I can assert that the author has not really studied Catholic theology very much. He is trying to promote an idea of "the sacred feminine" and claims the Church regards females and sexuality as dirty and inferior. He tries to turn Jesus into a mere mortal man, and Mary Magdalene into a divine God. He does not pay attention to the reverence given to women by the uplifting of Mary as Mother of God. The book talks about Eve bringing humanity into its downfall. But it says nothing of the ascension brought to humanity through the Blessed Virgin Mary and her cooperation with the salvific plan of God.
Having studied a lot of the issues that are touched on in "The Da Vinci Code," the alternate theory of Jesus and His supposed descendants, the novel did nothing to alter my beliefs in the orthodox teachings of Christianity. The release of this book spurned a huge reaction from Christians of all denominations. This lead to all kind of books debunking the claims made in Brown's novel, such as "The Da Vinci Hoax", "Debunking the Da Vinci Code", "The Da Vinci Fraud", etc. I think the dialogue is a good one, and I will suggest that you do take the time to read Dan Brown's novel. You also should look into some of the opposing claims made in books that criticize and argue against the main premise of the Code.
At the very least, reading books like this can demonstrate how wild alternate versions of history can be concocted and developed. There are stories that can convince lots of people of something that just isn't true or never happened. Brown says Jesus was married to Magdalene and had children. I say someone made it up. Even in the study of history, it is important to know that lies or fabrications can be made up. These lies can be meant either to tear down ideas one does not like, or to promote and build up ideas that one wants to become fact.
You can purchase "The Da Vinci Code" on Amazon here.