The allegory depicts Dante's journey through the depths of Hell. He is led by the Roman poet Virgil down into the nine circles of Hell, each of which holds and punishes progressively worse sinners. From the 'First Circle', where unbaptized souls live in peaceful limbo, down to the 'Ninth Circle', where Satan is trapped in ice, Dante sees firsthand the consequence of unrepentantly sinning against God.
"Through me, you go into a city of weeping; through me, you go into eternal pain; through me, you go amongst the lost people."
-Dante Alighieri, The Inferno