Angels and Demons (or "Dan Brown Thriller v.1.0") was the first novel to introduce the world to Dan Brown's storytelling method.
The plot is a fairly typical Dan Brown conspiracy plot. There's an ancient brotherhood known as "The Illumanati," a group of brilliant artists and scientists who strive for enlightenment. In Angels and Demons, they're looking for revenge against the church, as they are, apparently, still angry about the scientists that were tortured and killed during the Catholic Church's "Shut the fuck up, scientists, or we'll kill you for heresy" phase. The Illuminati kill the Pope at the book's beginning and have a plan to blow up the entire Vatican, proving that the Illuminati can hold a grudge longer than pretty much anyone.
"My name is Galileo Galilei. You killed my friends. Prepare to die."
Then, of course, our dashing symbologist and his ally Vittoria Vetra arrive on the scene. Vittoria's adoptive father has been killed by a mysterious Hassassin so she joins forces with Langdon to find him. Also, yes, the Hassassin's name is a reference to hashish - apparently, the word "assassin" comes from "Hassassin," as the killers of old would smoke marijuana before completing a contract. We assume they spent hours looking at their own hands and asking, "Dude, have you ever wondered what our bodies look like on the inside?" to psyche themselves up.
"Yeah, we'll kill your man for you. In exchange for Cheetos. And Mountain Dew."
So, the Hassassin plants a top-secret bomb-type thing (not specified because of spoilers) under the Vatican and goes around killing off cardinals right before the election of the new Pope, because you apparently need the best killer ever to defeat 90-year-old men. Langdon tries to follow him by figuring out The Path of the Illuminati, an initiation ceremony designed to make prospective Illuminati demonstrate their intellect by finding their way to the top-secret Illuminati lair. They do this by following hints to solve the mystery over a matter of weeks. The Path of the Illuminati was designed to be super-hard so the Catholic Church could never figure it out.
Langdon solves The Path in one night, by the way.
He manages to find the anti-matter bomb (we were lying when we said we wouldn't reveal the bomb-type thing a second ago) in time to save everyone, parachutes to safety from a helicopter using a one-yard-square parachute - which has been badly foreshadowed earlier in the book - and stops the true villain, who is revealed in an M. Night Shyamalan-worthy twist.
Then he gets laid. Dan Brown wanted to take the "happy ending" thing literally.
Dan Brown describes Robert Langdon as thin and attractive. Central Casting chose Tom Hanks.
Robert Langdon: The book's protagonist. Langdon is a Harvard symbology professor (the field of "symbology" doesn't actually exist, by the way) in his forties. He is an attractive, fit man known as "The Dolphin" due to his easygoing nature and the 50 laps a day he swims in the Harvard Pool. Dolphins are also known as the species that murders and rapes other sea creatures just for fun. Just sayin'.
Luckily, the Casting Director certainly got Vittoria Vetra right.
Vittoria Vetra: A young Italian marine biologist who uses fish to disprove Einstein's theories. Or something. She is described as being "smoking fucking hot." She becomes Langdon's ally during their quest to stop the Illuminati. Then, at the novel's very end, she and Langdon have sex, despite their being absolutely zero hint towards romance brewing between them anywhere in the last 400 pages. Her feeling seems to be that Langdon deserves sex as some kind of reward for saving the day. (We're totally okay with the sex-as-reward school of thought, by the way. Ladies)
Clearly, the Casting Director mixed up the Hassassin and Robert Langdon
Hassasin: Kills a ton of defenseless old men, buys and abuses expensive prostitutes, and abducts Vittoria because he wants to rape her. Clearly, when Dan Brown sets out to make an evil villain, he doesn't fuck around on the subtlety front.