In the prologue of the novel, a person runs away from his pursuers and rushes to the highest tower of Florence. Before his death, he thinks about his gift to mankind - Inferno.
The Professor of History of Culture, an expert in symbols named Robert Langdon wakes up in an unknown hospital. According to the outline of buildings from the dark window, he realizes that he is in Florence, although his last memories are of Harvard University. With the help of a doctor Sienna Brooks, he finds out that the last two days vanished from his memory. To Siena’s words amnesia is associated with traumatic brain injury and will soon be over.
A woman arrives at the hospital; she is an agent of a secret organization of the Consortium. She is sent to finish with Langdon. Agent rushes into the chamber, but Siena has led Langdon out through the emergency exit. The agent shoots, but Langdon and Sienna have time to leave by taxi.
The chef of Consortium expects news. His organization arranges various falsifications, forms false public opinion, and hides clients from the law. At this time, he has contacted with the wrong client, thus destruction threatens the Consortium. However the client, who recently had threw off the Florentine tower, left clear instructions that the boss is going to perform. One of the instruction is on the appointed day to circulate on major television channels video where a masked man tells about the Inferno, and take away from the deposit box a bone cylinder. However, the cylinder is kidnapped, and the chef sends an agent to rectify the situation.
Siena brings professor home. Langdon finds in Siena’s apartment articles about the girl-child prodigy cut from newspaper. Obviously, this girl is Siena. She shows Langdon capsule for the transport of dangerous substances, which has been sewn in the lining of his jacket. Langdon finger’s imprint opens the capsule. The professor calls to the US Consulate, and soon Siena sees the female agent through the window. Langdon decided that the US government wants to kill him. For what? The answer can be in the container and the professor opens it. Inside is a cylinder of carved bone, turned into the mini-projector. It projects a picture of Botticelli "Hell Map", which shows a part of the "Divine Comedy" by Dante.
The professor notices that the picture has changed, and there appears an inscription on it: “Truth can only be seen through the eyes of death”. To Siena’s home an armored car drives, people in uniform begin to look for Langdon. They are from Support for surveillance and response, and they are also looking for Langdon. Siena again saves the professor whom she likes more and more.
A female agent tracks down Langdon from the roof of a nearby building, she learns that the Consortium dismissed her from work. Frightened, she decides to rehabilitate and to complete the task.
Since the puzzle in the picture is associated with Dante, Langdon decided to go to the Old Town of Florence, where the poet was born and raised. The agent follows them. Langdon and Siena see that the gates of the Old Town are guarded by police pickets - obviously police are looking for them. They go in the vast gardens of Medici. While they are trying to be found via a radio-controlled helicopter with a camera, Langdon realizes what is encrypted on the projection of the picture. The tip points to a mural and a museum where it is set, is in the Old Town.
Director of the World Health Organization epidemiologist Elizabeth Sinskaya recalls her meeting with the brilliant scientist-geneticist. He predicted that the overpopulation of the planet would lead to the destruction of mankind, and believed that "human herd" is to be thinned out by a plague epidemic. Sinskaya understands that humanity will soon deplete the planet's resources, but cannot agree with the methods offered by the scientist. However, he is assured that the birth follows death, and to attain Paradise, a person has to go through hell.
Langdon and Siena are surrounded by police. They get from the trap with the erudite professor’s knowledge of the architectural features of the Italian palaces of the Medici time. But it is not possible to deceive the Consortium Agent - woman follows them closely. Langdon gets to the murals, where he is recognized by the museum caretaker. She says that the professor was here yesterday with his friend, the Italian art critic Busoni, they observed the death mask of Dante. Langdon realizes what the phrase means.
When Langdon asks to show him the mask, it turns out that it was stolen. Surveillance camera shot as Langdon and Busoni stole the death mask. From the custodian Langdon learns that the mask is the property of a scientist Bertrand Zobrist. Siena knows his demographic apocalypse theory. The supervisor calls the police. Langdon does not remember where the mask is, and cannot return it. The supervisor calls the secretary of Busoni, and discovers that the night before he died of a heart attack. Before his death, he left a message: “What you are looking for is safely hidden. The gates are open for you, but you hurry. "Paradise" Twenty-fifth.”
This is an allusion to the last part of "The Divine Comedy" by Dante. The museum is surrounded by Bruder’s people, but Langdon and Sienna again avoids the trap and go on a search for the mask. The agent follows them and dies. On their way Siena tells Langdon about Zobrist and his desire to use the genetics not for healing, and for the destruction of people. After meeting with Sinskay, the scientist became an outcast and threw himself off the Florentine tower. Siena believes that Zobrist is right.
Busoni’s tip leads Langdon to the old city Christening where Dante has been baptized. They are followed by a man with his face covered with a rash. They distract the guard's attention, get into the baptismal, the main gates of which are not locked, and find the mask. Its inner side is covered with primer for a canvas. After cleaning it, Langdon discovers verses that mention the treacherous Doge, a museum of sacred wisdom and underground palace.
In the baptismal, Langdon and Siena are overtaken by a person with a rash on the face. He introduces as Jonathan Ferris, of the WHO staff, and states that the professor is working for them. Siena is inclined to trust him, Langdon does not remember anything. Verse on the mask leads to Venice, where they are sent together previously knocking off the trail.
In a separate compartment of a train, Ferris says that Sinskaya attracted professors to the mystery and revealed the bone cylinder, which she removed from the Zobrist’s banking cell. Now, Langdon realizes that he is looking for the source of the plague, and his shrewdness determines the fate of mankind.
The chef of the Consortium changes his rules and views the video left by Zobrist. What he saw frightened him, and the chief calls the agent FS-2080, who is next to Langdon. It was the agent FS-2080, who was related to Zobrist, advised him to apply to the Consortium. The whole year the organization was hiding the scientist from the WHO, and he was creating the virus. He chef contacts with Sinskaya.
The FS-2080 is thinking about the deceased beloved, who was not only a lover but also a teacher. He believed that evolution should be artificially accelerated. The agent wants to bring his case to the end. In Venice it turns out that Langdon had made a mistake, the museum and the tomb of the Doge are located in Istanbul. The chef, Sinskaya and Bruder also fly to Venice. Langdon does not know that the NDP is a division of the World Health Organization. He tries again to escape from Bruder, but only Siena manages to get away.
Langdon meets Sinskaya and the chef. They show him the Zobrist’ video where the plastic bag lowered into the water is seen. When the bag burst, its contents fall into the water and contaminate it. Langdon learns that Siena was the mistress of Zobrist and Consortium’s agent. Siena, being a prodigy, could not find her place in life. She wanted to save the world, but knew how to do it, only when she had met Zobrist. Having to hide from the WHO the scientist left Siena, she resorted to the help of the Consortium, but found him too late. She watched as her beloved committed suicide.
Langdon’s brain injury is a fiction. The Consortium with medication provoked Langdon's amnesia and devised false memories. All this was done so Langdon began to trust Siena and got back the projection. Siena also used his knowledge to find the source of the plague first. Langdon likes Siena, he cannot come to his senses.
By WHO’s plane Langdon gets to Istanbul. On the plane he sees Ferris, who is also an employee of the Consortium. In Istanbul they found an underground room with a lake, which turned out to be the old city’s reservoir, where tourists are now allowed. Shrewd Siena follows them.
Langdon's efforts are in vain: the bag dissolved, the infection occurred. Seeing in the underground hall Siena, Langdon chases after her. She can run, but stay - she had nowhere to run. Siena tells Langdon about the Zobrist’s letter, which she received before the disappearance of the scientist. Zobrist wrote her about invented virus that invades into human genetic code, and causes infertility. He loved mankind. Not wanting to kill millions of people, he came up with a safe alternative of the plague.
Siena was afraid that people would understand the principle of the virus, and would produce biological weapons. She decided to destroy the virus, but it was too late. Day, marked by Zobrist, was not a period when the virus will be released, but the date by which all humanity will be infected. The chef realizes that Sinskaya will not let him go unpunished. He organizes a hoax and tries to escape, but he did not succeed – he is arrested. Sinskaya tries to prevent panic. Faster than virus spreads only fear.
Langdon leads Siena to the doctor Sinskaya. She tells about the virus, which has infected the third of the population of the earth. Zobrist’s letter is destroyed, but Siena has an unique memory, and Langdon convinces Sinskaya to talk with the girl. She recognizes the truth of Zobrist and agrees to work with Siena.
Sinskaya takes Siena away for a medical forum in Geneva. Langdon escorts them. After the kiss with Siena, the professor hopes that they have a future.
