In the book, Robert Langdon and Vittoria Vetra are summoned to Vatican City by the Swiss Guard, who need them to locate the antimatter counting down within their walls and discover the secret behind the Illuminati ambigram. When Langdon requests entry to the Vatican Archives, he is led to the Camerlengo, who is currently the only person in the world allowed to grant them entry. As the four preferiti are murdered by the Hassassin, Janus, an Illuminati master, contacts the killer and gives him his orders.
As Langdon walks the Path of Illumination in an attempt to find the Illuminati lair, Carlo's history is slowly revealed as the story progresses. As a child, he never knew his father. His mother was a woman named Maria, whom he loved more than anybody else in the world. One day, she was tragically killed in an explosion, and Carlo was forced to work in the army. As he could not bear to kill another human being, he was given the job of a medical pilot. When he was still young, the Pope adopted him from the army and made him Camerlengo. However, shortly before his death, the Pope told him a secret which sickened Carlo and left him crying on his floor.
As the public were told that the Pope died from a stroke, the Hassassin contacts them and claims that the Illuminati were responsible for the death and poisoned him. When Vittoria confirms that he may have been overdosed on his own medicine and that the result would be very visible, she and Carlo visit the Pope in his sarcophagus and check his body; he has a completely black tongue, meaning he was poisoned as an effect of the overdose was the tongue turning black.
Later, while Conclave is in progress inside the Sistine Chapel, Carlo enters, much to the shock of the cardinals - especially as he has brought with him two news reporters: Gunther Glick and Chinita Macri. Being viewed by the whole world via Macri's camera, he informs the public of the cardinals' murder, the Pope's poisoning and the antimatter threatening to destroy the Vatican. He then gives an incredibly emotional speech about the war between science and religion and the damage the former has done to the planet's wonders. Through this address, the Camerlengo touches the hearts of billions of people and becomes a respected icon who the world believes as the only possible saviour of Christianity.
As the news of the antimatter and the cardinals' murder shocks the public, rumours of the coming of an '11th Hour Samaritan' with information about the situation go around Rome and the Vatican.
When Langdon completes the Path in search of Vittoria (who was kidnapped by the Hassassin during the death of the third cardinal) and travels to Castel Sant'Angelo, the Hassassin tells him that the final victim of the Illuminati's plot will be the Camerlengo himself, and that the Samaritan is actually Janus, who will be the killer. Langdon and Vittoria defeat the Hassassin and escape to find the Camerlengo.
The Samaritan lands, and it turns out to be Maximillian Kohler, the man who sent them to Vatican City in the first place. Fearing that Kohler is Janus, Langdon and Vittoria head through el passetto, a secret passage running between Castel Sant'Angelo and the Pope's library. Rocher, a high-ranking Swiss Guard, escorts Kohler to the Pope's Office to see the Camerlengo, and allows him to enter alone while he, several others and Lieutenant Chartrand, another Guard, stand outside the door.
Langdon and Vittoria arrive at the library and are let in by Chartrand, but as they enter they hear Carlo screaming. The soldiers kick the door down and find the Camerlengo lying on the floor with an Illuminati brand on his chest, with Kohler standing by him with a gun. Instinctively several guards open fire on Kohler, who falls backwards into his wheelchair. When Carlo accuses Rocher of being an Illuminatus, the latter runs at him only to be gunned down by Chartrand. As Kohler lies dying in his chair, he hands a video camera to Langdon and tells him to give it to the media. The brand on Carlo's chest turns out to be the Illuminati Diamond, the most sacred and wondrous of them all.
Langdon and the Guard carry the screaming Camerlengo out into St Peter's Square to load him into a medical helicopter. While being viewed by thousands of people in the Square as well as people all over the world due to the media's hot pursuit, Carlo slips away from them and begins to cry out, claiming that he has received a message from God and that he now knows the location of the antimatter. He runs back into St Peter's Basilica, through the Niche of the Palliums and into the Necropolis towards St Peter's tomb. Despite being begged not to enter by Langdon and the others, he still enters, and they do indeed find the antimatter on top of the tomb.
The Camerlengo hurries back out into the Square with the antimatter and climbs into the helicopter, intending to take the antimatter up into the air and leave it there to explode out of range of the ground. Langdon misinterprets Carlo's idea and, believing that he plans to throw it into a desolate area, climbs in with him as he flies away.
High up in the sky, the Camerlengo puts the helicopter into autopilot and leaves the antimatter inside. He bids farewell to Langdon, and is sorry for his misinterpretation, saying that he inadvertently made the 'ultimate sacrifice'. With that, he bails out with the only parachute in the copter, and flies down onto the Basilica. Up in the sky, the antimatter explodes, obliterating the helicopter (and supposedly Langdon) but narrowly missing the ground. As the entire world thanks and cheers for him, Carlo simply kneels on the roof of the Basilica and quietly prays.
Hailed as a hero by Catholics all over the planet, the Camerlengo becomes the most likely candidate for the new Pope. He is cleaned and revitalised by his servants before being summoned to the Sistine Chapel by Cardinal Mortati and the others. Upon arriving, however, he is horrified to discover Langdon at the altar.
Having survived the explosion by leaping out of the copter and using the cockpit's screen cover to slowly control his fall and land in the Tiber River, Langdon had listened to the conversation recorded on Kohler's camera and hurried over to the Chapel to show the cardinals. Now, the College of Cardinals knows the awful truth.
The video on the camera is a view of the meeting between Carlo and Kohler in the Pope's Office. A short while ago, the Pope, who was very interested in bridging the gap between science and religion, had attended an audience of Leonardo Vetra (Vittoria's father and the founder of the antimatter). Intrigued by the idea of using science to explain God's existence, the Pope became involved in Leonardo's plans, much to Carlo's dismay. Worried that Vetra's latest discovery would guarantee science's victory over religion, Carlo adopted the disguise of Janus and ordered the Hassassin to kill Leonardo, steal the antimatter and place it on the tomb.
Back in the Vatican, Carlo had expressed his anger to the Pope, who told him a secret that he had never told before: he had fathered a child, forcing Carlo to believe he had broken his vows as a holy man. This was the secret that had sickened Carlo, and before the Pope could fully explain, he had run out of the room in shock. Incredibly stressed by all the recent things the Pope had done, Carlo had poisoned the Pope that night and killed him, leading to the events occurring in the present.
After Kohler had revealed to Carlo that he knew the truth, Carlo had branded himself with the Illuminati Diamond in order to make it look like Kohler was really Janus. Knowing that Rocher had also been informed of the truth and was acquainted with Kohler, Carlo had him killed as well to completely clear out those who knew the truth; with all of them dead, he could finish off his more spiritual and reasonable plan - but he had not suspected that Langdon would find out too.
In the Sistine Chapel, the devastated Camerlengo tried to explain his actions to Langdon, Vittoria and the College; he only wished to make people believe again, but did so through an act of terrorism. He wanted to use the Illuminati situation to frighten the people of the world, then pretend that God had manipulated the scene to save the Vatican.
Cardinal Mortati steps forward and reveals the final twist unbeknownst to even the Camerlengo. When the recently deceased Pope had been a candidate in the College of Cardinals years ago, Mortati had been the Devil's Advocate: the man who is tasked with checking each candidate's background to make sure they are eligible. While he was checking the man who would become the Pope, he discovered that he was in love with a young nun named Maria.
The Pope and Maria were desperate to be parents, but they did not wish to break their vows via sexual intercourse. However, this was when artificial insemination had just been invented. The two saw it as a miracle, and used it to have a child: Carlo Ventresca. When Carlo was very young, the Pope's duty called him away, and Maria was left to mother Carlo by herself.
After the explosion which had killed Maria, the Pope feared that Carlo had also been killed. When he learned that he was alive and well, he was overjoyed and adopted him from the army.
Upon hearing this news, the Camerlengo falls silent. Vittoria wishes to reveal this to the public, but Mortati urges her not to; Carlo's views were acceptable, only his actions were not - if the people heard that the Camerlengo was really behind the attacks, they would possibly never believe in anything holy again. When they turn around, however, Carlo has fled.
In an immense surge of guilt and grief from causing the deaths of so many, let alone his own father, Carlo heads into the Basilica and takes an oil lamp from the Niche of the Palliums. He stands on the balcony over St Peter's Square - and as the public calls out to him, he immolates himself with oil from the lamp in front of the massive crowd. Mortati comes in later and collects his ashes.
Mortati has just been elected as the new Pope, but it turns out that the Camerlengo had actually been elected first via a method known as 'election by adoration'. This occurred with Carlo when everybody was calling out to him after his landing on the Basilica.
Mortati places Carlo's ashes in a golden urn and places it in the late Pope's sarcophagus, stating that: "There is no love greater than that of a father for his son".