In the beginning of the novel Professor Robert Langdon wakes up in a hospital with a head wound and no memory of the last few days. His last memory is walking on the Harvard campus, but he quickly realizes that he is now in Florence. Sienna Brooks, one of the doctors tending to him, tells him he suffered a concussion from being grazed by a bullet and had stumbled into the emergency ward. Suddenly, Vayentha, a female assassin who has been following Robert, breaks into the hospital, shoots the doctor in charge of Robert's care, and approaches Robert's room. Sienna grabs Robert and they flee to her apartment. While at Sienna's apartment they discover a hidden pocket in his jacket which contains a biohazard container containing a a small medieval bone cylinder fitted with a hi-tech projector that displays a modified version of Botticelli's Map of Hell. Suddenly soldiers breaking into Sienna's apartment building forcing them to flew. Robert and Sienna head toward the Old City, believing the cylinder must have something to do with Dante, however, they meet by Venice police and the same soldiers and forced to sneak though most of the city. After spending some time in venice Langdon and Sienna now posses Dante's Death Mask which Zobrist used to hide a riddle on the back of. the Florence Baptistry, where he and Sienna find the Dante mask along with a riddle from its current owner, a billionaire geneticist named Bertrand Zobrist. Sienna later explains that Zobrist was a geneticist who advocated the halting of humanity's growth, due to its out of control population. And that he was rumored to be working on a means to do so using an engineered disease. A man named Jonathan Ferris, with a large bruise on his chest which he hides from the two, and a severe rash on his face, claiming to be from the World Health Organization (WHO), comes and helps them escape the soldiers. They follow the riddle to Venice, where Ferris suddenly falls unconscious, with Sienna claiming he is suffering from massive internal bleeding, causing Langdon to suspect Ferris has been infected with Zobrist's plague. Robert is captured by a group of black-clad soldiers while Sienna escapes.Robert is taken to Elizabeth Sinskey, the director-general of the WHO, and is given an explanation of what is going on: Zobrist, who committed suicide the week before, was a brilliant geneticist and Dante fanatic who has supposedly developed a new biological plague that will kill off a large portion of the world's population in order to quickly solve the problem of the world's impending overpopulation. Elizabeth raided Zobrist's safe deposit box, found the cylinder, and flew Robert to Florence to follow the clues. However, Robert stopped communicating with Elizabeth after meeting with Marta and Ignazio, and the WHO feared he betrayed them and was working with Zobrist to unleash the plague. The soldiers were the WHO's emergency response team and never meant to kill Robert. Zobrist had paid a shadowy consulting group called The Consortium to protect the cylinder until a certain date. He also left a disturbing video filled with Dante imagery, which also showed a picture of the plague itself, kept in a hidden underwater location, within a slowly dissolving bag. The video claims that the world will be changed the following morning. When Elizabeth took it away, they were obligated to protect whatever the bone cylinder pointed to. They kidnapped Robert after the meeting with Marta and Ignazio, but Robert hadn't yet solved the whole riddle. They gave Robert benzodiazepine drugs to erase his short-term memory, created a fake head wound, and staged every event up to this point so that Robert would be motivated to solve it. Sienna, Vayentha, and Ferris are all actors working for The Consortium. Sienna goes rogue, and The Consortium realizes she was a secret supporter and lover of Zobrist. She learned where the plague is being kept after Robert solved the riddle and acquires a private jet to get to it before everyone else. Robert, the WHO, and The Consortium, team up to stop her. From watching Zobrist's video they conclude that the bag containing the plague will be fully dissolved by the date the video specifies, and that Zobrist's clues point to its location: the Hagia Sophia in Istanbul. However Robert and the others find the plague is in the Cistern where Sienna already is. Langdon spots her and chases her until she finally gives up, she then begins to sob and tells Langdon everything about the plague. She tries to run away but Langdon persuades her to stay so she can inform Sinskey. Sienna tells Sinskey everything she knows about Zobrist Vector Virus. After Sinskey hears everything she decides to let Sienna help her address the world and answer future questions that may arise from the world leaders. Langdon and Sienna say there goodbyes before she has to leave with Sinskey. They kiss, happy ending.