What happens in Canto XVIII of Dante's Inferno?

Canto XVIII
The monster Geryon has flown away, leaving his passengers at the top of the Eighth Circle of Hell. The sight, perhaps unsurprisingly, is simultaneously dreary and terrifying. Nude sinners march through the dim fortress; their unprotected bodies are relentlessly whipped by demons who herd them continuously along. Dante describes the scene as he and Virgil move forward:

"There is a place in Hell called Malebolge,
Wholly of stone and of an iron colour,
As is the circle that around it turns.
Right in the middle of the field malign
There yawns a well exceeding wide and deep,
Of which its place the structure will recount.
Round, then, is that enclosure which remains
Between the well and foot of the high, hard bank,
And has distinct in valleys ten its bottom.
…
Upon my right hand I beheld new anguish,
New torments, and new wielders of the lash,
Wherewith the foremost Bolgia was replete.
Down at the bottom were the sinners naked;
…
Beheld I horned demons with great scourges,
Who cruelly were beating them behind.
Ah me! how they did make them lift their legs
At the first blows!’

As Dante watches the grim procession, he is reminded of his nemesis, Pope Boniface VIII and the year of the first Jubilee, which began on February 22, 1300. On this day, the pontiff declared that the king was subordinate to the pope, in matters both spiritual and secular. Furthermore, Boniface arranged for the selling of indulgences during the year-long run of the Jubilee; anyone who visited a Roman Catholic church and made an “offering” was forgiven of their sins by the clergy. The response from the parishioners was overwhelming; so many streamed into Rome that soldiers were required to keep the crowds orderly and two lines moving steadily in and out of St. Peter’s Cathedral.
Dante watches as the tormented souls pass him; one of their number makes eye contact with the poet. Dante immediately recognizes the shade as Venedico Caccianemico, and asks the man what he has done to bring this torment upon himself.
Caccianemico reluctantly confesses that in life, he had sold his sister into sexual slavery:

"Unwillingly I tell it;
But forces me thine utterance distinct,
Which makes me recollect the ancient world.
I was the one who the fair Ghisola
Induced to grant the wishes of the Marquis,
Howe'er the shameless story may be told.”

For some reason, Caccianemico thinks that claiming others are guilty of his sin lessens his own culpability, so he tells other that others from his area, Bologna, are also perpetrators, ones who have said “sipa” (“yes”):

“Not the sole Bolognese am I who weeps here;
Nay, rather is this place so full of them,
That not so many tongues to-day are taught
'Twixt Reno and Savena to say 'sipa;'
And if thereof thou wishest pledge or proof,
Bring to thy mind our avaricious heart."

Dante cannot answer Caccianemico, however, because a demon viciously whips the sinner and the man is forced to move along. The demon shouts,

"Get thee gone
Pander, there are no women here for coin."

Dante and Virgil watch the line of sinners march, come to a rocky ridge, and then turn around, travelling toward the poets and back from whence they had come. Among their number, Virgil points out one man in particular: Jason of the Argonauts:

“See that tall one who is coming,
And for his pain seems not to shed a tear;
Still what a royal aspect he retains!
That Jason is, who by his heart and cunning
The Colchians of the Ram made destitute.
He by the isle of Lemnos passed along
After the daring women pitiless
Had unto death devoted all their males.
There with his tokens and with ornate words
Did he deceive Hypsipyle, the maiden
Who first, herself, had all the rest deceived.
There did he leave her pregnant and forlorn;
Such sin unto such punishment condemns him,
And also for Medea is vengeance done.
With him go those who in such wise deceive;
And this sufficient be of the first valley
To know, and those that in its jaws it holds."

Even though the man is lauded in literature, he also committed a very terrible sin. Virgil tells Dante that Jason abandoned Hypsipyle of Lemnos, after seducing her and getting her pregnant. With Hypsipyle not in the way, Jason was free to go steal the legendary Golden Fleece from Colchis, the land of great wealth that bordered the Earth and the Heavens.
The infamous Jason is now just another of the condemned souls. He moves along without a word to the travelers, spurred on by the whip of his tormentor.
Virgil and Dante walk until they come to a fetid pool. Here, sinners are immersed in excrement. The shades fight among themselves in the mire, and their complaints become mold on their filthy bodies. Edging closer, Dante sees how tremendously deep the moat is in which they are punished; he comes close enough to see some of their smeared faces. One of the condemned espies Dante looking at him in particular and cries out:

“Wherefore art thou so eager
To look at me more than the other foul ones?"

Dante calls the man by name: Alessio Interminei of Lucca (There are few details about this man’s life other than that he was a Guelph). The shade hits himself over the head, admits that in life he had been guilty of the sin of flattery, and sinks back down into the excrement.
Virgil then points out one more sinner in the muck, a woman whom he identifies as Thais, a harlot whose sin was being gratuitously thankful to her lover for physical pleasure. Now, immersed in Malebolge, Thais relentlessly scratches at her torn flesh with her filth-encrusted nails.
Disgusted with the sight of her, Virgil stops talking and the two move wordlessly on.