The book contains several ambigrams created by real-life typographer John Langdon.1 Besides the "Angels & Demons" and "Illuminati" designs, the title of the book is also presented as an ambigram on the hardcover book jacket, and on the inside cover of the paperback versions. The book also contains ambigrams of the words Earth, Air, Fire, and Water, which has served to bring the art of ambigrams to public attention by virtue of the popularity of the book.2 The "Illuminati Diamond" mentioned in the book is an ambigram of the four elements that are arranged in the shape of a diamond.3
Leonardo Vetra, one of CERN's top physicists who has discovered how to create antimatter, is murdered, his chest branded with an ambigram of the word "Illuminati", an ancient anti-religious organization thought extinct. CERN director Maximilian Kohler calls Vetra's adopted daughter, Vittoria, and Harvard University Professor Robert Langdon, an expert on symbology and religious history, for help. After determining the ambigram is authentic, they discover that a canister of antimatter from Leonardo's lab, which has been stolen, will explode in 24 hours when the canister's battery runs out. Langdon and Vittoria go to Vatican City, where four Preferiti, the Cardinals who are the favorite candidates for Pope, are kidnapped by the Hassassin, who plans to blow up the Vatican with the antimatter canister and kill the four cardinals under the orders of "Janus", the leader of the Illuminati.
Believing that the four cardinals will be ritually murdered on the four altars of the "Path of Illumination", Langdon and Vittoria follow a series of clues left in various churches in and around Rome. After finding the first two men dead (one suffocated by earth and another whose lungs were punctured), they confront the assassin in the act of murdering the third. However, they fail to save the third cardinal and, as the location catches fire, the assassin kidnaps Vittoria. Langdon also fails to save the last cardinal, who is drowned in the Fontana dei Quattro Fiumi, and confronts the assassin in the Castel Sant' Angelo, the Church of Illumination. Langdon frees Vittoria and together they send the assassin falling several hundred feet to his death.
Meanwhile, Kohler arrives to confront Camerlengo Carlo Ventresca, the late Pope's closest aide. Fearing Kohler is Janus, the two hurry back to St. Peter's Basilica. However, the Swiss Guards intervene and open fire on Kohler when Ventresca screams. Just before he dies, Kohler gives Langdon a mini video camera which records his confrontation with Ventresca. With Langdon in pursuit, Ventresca ventures into the catacombs and finds the canister atop the tomb of Saint Peter. Ventresca takes the canister to a safe height in a helicopter and parachutes safely just as the canister explodes in the sky. Reacting to this "miracle", the Cardinals debate whether to elect Ventresca as the new Pope. Meanwhile, Langdon learns from the video that Ventresca himself is Janus. He confesses that he poisoned the Pope upon the Pope's revelation that he had fathered a child, and opposes Vetra's attempt to bridge science and God. Under the guise of Janus, he recruited the assassin to kill Vetra, steal the antimatter, and kidnap and murder the Preferiti as part of a false flag attack to rally the Catholic Church behind himself and discredit scientific advancement of the kind which had made the antimatter weapon possible.
Cardinal Saverio Mortati, Dean of the College of Cardinals, reveals that Ventresca is in fact the late Pope's biological son, conceived with a nun through artificial insemination. Overcome with guilt, Ventresca soaks himself in oil and sets himself on fire before a crowd of onlookers in St. Peter's Square. Mortati is unanimously elected Pope by the Cardinals, and Langdon and Vittoria reunite at Hotel Bernini where they spend the night together. Mortati bequeaths the Illuminati's final brand to Langdon for his research, with the implicit request he restores it in his final will.
The book's first edition contained numerous inaccuracies of location of places in Rome, as well as incorrect uses of Italian language. Some of the language issues were corrected in the following editions.4
Aside from the explicit introduction, the book depicts various fictional experts explaining matters in science, technology, and history in which critics have pointed out inaccuracies. An example of this is the antimatter discussions, wherein the book suggests that antimatter can be produced in useful and practical quantities and will be a limitless source of power. CERN published an FAQ page about Angels & Demons on their website stating that antimatter cannot be used as an energy source because creating it takes more energy than it produces.5
Angels & Demons Decoded, a documentary on the American cable television network, The History Channel, premiered on May 10, 2009, shortly before the release of the novel's film adaptation. The documentary explores the various bases of the novel's story, as well as its inaccuracies. A CERN official, for example, points out that over the last 20 years, approximately 10 billionths of a gram of antimatter had been produced at the facility, whose explosive yield is equivalent to that of a firecracker, far less than is needed for it to be the threat depicted in the novel, where the device holds 0.25 grams.6
According to The Boston Globe language columnist Ben Zimmer, the position of Devil's Advocate, which is indicated in the novel to have a role in the selection of the pope, is in fact employed to present arguments against the proposed canonization of a person as a saint (which frequently is the case with former popes). While the background of papal candidates are presumably scrutinised, there is no particular office and not one bearing this title. Zimmer adds that the office was abolished by Pope John Paul II in 1983, 17 years before the novel was published.7 Further, election through acclamation, which plays a role in the climax of the novel, was in fact a method of electing the pope, but was abolished in 1996 by Pope John Paul II, four years before the novel.8
"Angels & Demons" Archived November 2, 2013, at the Wayback Machine. www.johnlangdon.net. Retrieved August 26, 2013. http://www.johnlangdon.net/angelsanddemons.php ↩
"The Ten Most Famous Ambigrams". Ambigram Magazine. April 20, 2009. Archived from the original on March 16, 2010. Retrieved April 21, 2009. https://web.archive.org/web/20100316151115/http://www.ambigram.com/most-famous-ambigrams ↩
"Angeli e Demoni di Dan Brown". Il Piacere Della Lettura. 2006. Retrieved August 26, 2013. http://leggiamo.altervista.org/thriller_angeliedemoni.htm ↩
"Can antimatter be used as an energy source?". CERN. Archived from the original on July 10, 2015. Retrieved August 3, 2015. https://web.archive.org/web/20150710081802/http://angelsanddemons.web.cern.ch/faq/antimatter-to-create-energy ↩
Comtois, David; Hartford, Scott (Writers). Angels & Demons Decoded. May 10, 2009. The History Channel. /wiki/History_(U.S._TV_channel) ↩
Martin, Rachel (March 3, 2013). "Who Is The 'Devil's Advocate'?" NPR. https://www.npr.org/2013/03/03/173350724/who-is-the-devils-advocate ↩
https://www.catholic.com/encyclopedia/acclamation-in-papal-elections https://www.catholic.com/encyclopedia/acclamation-in-papal-elections ↩