Wikipedia
At CERN, a small container of anti-matter (yes, I know) is stolen while the
word mourns the death of the Pope. Soon after, Robert Langdon is asked by the
Vatican police to help in a case of kidnapping of the four cardinals who are
the favourites in the upcoming election of the next Pope. The kidnapper
promises to kill them at an hour interval between 8PM and 11PM, and that at
midnight the antimatter would explode and destroy the Vatican and most of
Rome. The clues indicate that he belongs to the Illuminati, a society of early
scientists persecuted by the Church for their teachings and who has gone
underground since then. In the meanwhile, the remaining cardinals start the
conclave to elect the new Pope. Langdon deduces that the kidnapped cardinals
will be killed at the location of a secret path of four steps (related to the
four elements) across Rome that leads to the Illuminati's secret meeting
place. They also find a proof that the Pope had been murdered. Langdon,
Vittoria (a physicist from CERN who worked on the antimatter) and the police
arrive too late to save the first victim, who is found under the Chigi chapel.
They also arrive too late to save the second cardinal, murdered on St. Peter's
Square, and the third one, burned alive in Santa Maria della Vittoria, where
the kidnapper kills most of the policemen accompanying Langdon. In the
meanwhile McKenna the Camerlengo fails to convince the cardinals to break the
conclave and to evacuate the Vatican. Langdon manages to save the fourth one,
Baggia, from being drowned in the Fontana dei Quattro Fiumi, who reveals they
were kept prisoner in the Castel Sant'Angelo. Langdon and Vittoria go there
with the police but get separated from them. The discover that McKenna is
meant to be the next victim and they return to the Vatican through an ancient
secret passage. They arrive just in time in the Pope's apartments to see
McKenna threatened by Richter, the chief of the Swiss guard, who get killed by
the police. Langdon then deduces that the antimatter is hidden in St. Peter's
tomb. As only a few minutes remain before midnight, McKenna, having been
trained in the Irish Air Force, takes the container and flies it up with a
helicopter before jumping with a parachute. The explosion happens in high
altitude, and the Vatican suffers only minor damage. The cardinals then
consider making McKenna the new Pope, despite the fact he's not a cardinal.
Just before dying Richter had given Langdon a key to access video recordings
of the Pope's apartments, where Langdon discovers that the kidnapping has been
organised by McKenna with the purpose of becoming the next Pope; the
Illuminati is only a convenient common enemy to unite the Church against the
perceived blasphemy of reconciling science and religion. Seeing no issue,
McKenna commits suicide and the conclave elects Baggia as the new Pope.