The poem starts on Maundy Thursday in the year 1300. The narrator, Dante himself, is thirty-five years old, and thus "halfway along our life's path" (Nel mezzo del cammin di nostra vita) – half of the Biblical lifespan of seventy (Psalms 89:10, Vulgate). The poet finds himself lost in a dark wood (selva oscura) in front of a mountain, assailed by three beasts (a lion, a lonza [usually rendered as "leopard" or "leopon"], and a she-wolf) he cannot evade. Unable to find the "straight way" (diritta via, also translatable as "right way") to salvation, he is conscious that he is ruining himself and falling into a "deep place" (basso loco) where the sun is silent (l sol tace).
Dante is at last rescued by the Roman poet Virgil, who claims to have been sent by Beatrice, and the two of them begin their journey to the underworld. Each sin's punishment in Inferno is a contrapasso, a symbolic instance of poetic justice; for example, fortune-tellers have to walk forward with their heads on backward, unable to see what is ahead, because they tried to see the future through forbidden means. Such a contrapasso "functions not merely as a form of divine revenge, but rather as the fulfilment of a destiny freely chosen by each soul during his or her life."
Dante passes through the gate of Hell, which bears an inscription ending with the famous phrase "Lasciate ogne speranza, voi ch'intrate", most frequently translated as "Abandon all hope, ye who enter here."
Before entering Hell completely, Dante and his guide see the Uncommitted, souls of people who in life did nothing, neither for good nor evil; among these Dante recognizes either Pope Celestine V or Pontius Pilate (the text is ambiguous). Mixed with them are outcasts who took no side in the Rebellion of Angels. These souls are neither in Hell nor out of it, but reside on the shores of the Acheron, their punishment to eternally pursue a banner (i.e. self interest) while pursued by wasps and hornets that continually sting them as maggots and other such insects drink their blood and tears. This symbolizes the sting of their conscience and the repugnance of sin. This may also be seen as a reflection of the spiritual stagnation they lived in. As with the Purgatorio and Paradiso, the Inferno has a structure of 9+1=10, with this "vestibule" different in nature from the nine circles of Hell, and separated from them by the Acheron.
After passing through the "vestibule," Dante and Virgil reach the ferry that will take them across the river Acheron and to Hell proper. The ferry is piloted by Charon, who does not want to let Dante enter, for he is a living being. Virgil forces Charon to take him by means of another famous line: Vuolsi così colà dove si puote, which translates to, "So it is wanted there where the power lies," referring to the fact that Dante is on his journey on divine grounds. The wailing and blasphemy of the damned souls entering Charon's boat contrast with the joyful singing of the blessed souls arriving by ferry in the Purgatorio. The passage across the Acheron, however, is undescribed, since Dante faints and does not awaken until he is on the other side.
Virgil then guides Dante through the nine circles of Hell. The circles are concentric, representing a gradual increase in wickedness, and culminating at the centre of the earth, where Satan is held in bondage. The sinners of each circle are punished in a fashion fitting their crimes: each sinner is afflicted for all of eternity by the chief sin he committed. People who sinned, but prayed for forgiveness before their deaths are found not in Hell but in Purgatory, where they labour to be free of their sins. Those in Hell are people who tried to justify their sins and are unrepentant.
Allegorically, the Inferno represents the Christian soul seeing sin for what it really is. What the three beasts may represent has been the subject of much controversy over the centuries, but one suggestion is that they represent three types of sin: the self-indulgent, the violent, and the malicious. These three types of sin also provide the three main divisions of Dante's Hell: Upper Hell (the first 5 Circles) for the self-indulgent sins, Circles 6 and 7 for the violent sins, and Circles 8 and 9 for the malicious sins. The sins may also be split into sins of incontinence (upper hell), named so because of their passionate and uncontrolled nature, and sins of malice (lower hell), where the sins are premeditated.