Robert Langdon, a professor of religious symbology at Harvard University, receiving an urgent late-night phone call requesting his expertise at the European Organization for Nuclear Research (CERN) in Switzerland. Langdon is met by Maximilian Kohler, the director of CERN, who informs him that a physicist named Leonardo Vetra has been murdered and branded with an ancient symbol associated with the Illuminati, a secret anti-Christian brotherhood thought to be extinct for centuries.

Kohler explains that he called Langdon because of his knowledge of the Illuminati based on research for Langdon's book. Langdon recounts the history of the Illuminati - a group of scientists in 16th century Italy who opposed the Catholic Church's inaccurate teachings and persecution of science. The group was founded by Galileo Galilei, who tried to reconcile science and religion. However, the church hunted the Illuminati ruthlessly, forcing them underground where they grew increasingly anti-Christian over the centuries. The Vatican denounced them as Satanic.

Langdon and Kohler examine Vetra's body, which has been stripped naked and branded on the chest with the word "Illuminati" written as an ambigram that can be read upright and inverted. Langdon is amazed because, although legends of the Illuminati symbol exist, no academic has ever seen it. Kohler tasks Langdon with using his expertise to shed light on the murder.

Meanwhile, in another country, a killer known as the Hassassin meets with his employer, Janus, who says the first phase of their plan is complete after Vetra's successful murder and branding. The Hassassin is told to rest before the next phase, but he decides to visit a brothel as a reward.

Back at CERN, Langdon explains to Kohler that the brand suggests the long-lost Illuminati have resurfaced to exact revenge against the Catholic Church. He says this murder implies the Illuminati have infiltrated the deepest levels of the church. Kohler and Langdon realize they may be dealing with an extremely dangerous and influential adversary.