The poem begins on the day before Good Friday in the year 1300. The narrator, Dante Alighieri 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 life expectancy of seventy (Psalm 90:10). The poem finds him lost in a dark wood, assailed by three beasts (a lion, a leopard, and a she-wolf) he cannot evade, and unable to find the "straight way" (diritta via)—also translatable as "right way"—to salvation. Conscious that he is ruining himself and that he is 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, 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 forwards with their heads on backwards, unable to see what is ahead, because they tried, through forbidden means, to look ahead to the future in life. 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, the ninth (and final) line of which is the famous phrase "Lasciate ogne speranza, voi ch'intrate", or "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 while maggots and other such insects drink their blood and tears. This symbolizes the sting of their conscience and the repugnance of sin. 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 ove 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 are a contrast to the joyful singing of the blessed souls arriving by ferry in the Purgatorio. However, the actual passage across the Acheron is undescribed since Dante faints and does not wake up 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. Each circle's sinners 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, and the three beasts 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.
For more information go to Divine Comedy.