Midway through the course of his life, Dante wakes up in a dark forest, having lost his way from the right road. He does not know how he strayed from the road or how he arrived in the woods, but he says that he will record his story.
Having strayed from the right, virtuous path of life, Dante finds himself in a dark landscape of ambiguity, confusion, and possible sin. He narrates his own story both for the reader's benefit and for his own everlasting fame. It is unclear to what degree this forest is a real, earthly place or a more allegorical, spiritual landscape.
Dante sees a mountain with the sun shining above it. The sight comforts him, and he attempts to climb the mountain. But as he begins his climb, a leopard leaps in front of him, forcing him to turn back. Dante is still hopeful that he can climb the mountain, encouraged by the bright rays of the sun. But then a terrifying lion comes into his path, followed by a fierce wolf.
Dante attempts to find an easy path to the goodness and clarity suggested by the shining sun. However, his way is blocked by three fierce animals, which represent sin. (The three animals can potentially be seen as standing in for the three kinds of sin: lack of self-control, violence, and fraudulence or deception.)
Dante is frightened by the animals and loses all hope of scaling the mountain. He reluctantly returns to the dark forest, where he sees some kind of figure. He calls out to it, unsure if it is a man or a ghost. The figure identifies himself as the shade of Virgil, the greatest poet of ancient Rome. Dante is awe-struck and impressed, calling Virgil his master.
Despite Virgil's paganism (i.e. the fact that he is non-Christian, having lived before Christ), Dante reveres him as a good man and an important poetic model, admirable both for his virtuous life and for his skill with words.
Dante tells Virgil about how he was turned back from ascending the mountain by wild beasts, and Virgil informs him that he must take a different path. He says that the wolf prevents anyone from passing, and will continue to do so until a greyhound comes and drives her away, hunting her back to hell.
Virgil quickly assumes the role of Dante's spiritual and literal guide. The greyhound represents the coming of Jesus on Judgment Day to drive away the forces of sin.
Virgil says he will guide Dante on his journey. He says Dante will go through a terrible place with souls in torment, after which "a worthier spirit" (1.121) will lead him on the rest of his journey toward heaven, since God will not allow Virgil, a pagan, to enter heaven. Dante agrees to follow Virgil on this journey.
In spite of Virgil's admirable character, he is still a pagan and cannot guide Dante past hell, where he is confined. In order to reach the light of heaven, Dante must journey through the darkness of hell, where he will see the consequences of sinning. Dante's journey will take him from our world to that of the afterlife.