What happens in Canto XIV of Dante's Inferno?

Canto XIV
Before Dante and Virgil take leave of the Florentine who has committed suicide and, as punishment, has been transformed into a thorny bush, Dante takes pity on the shade. He gathers up all the broken branches he is able to find and gently places them back on the tangle of twigs.
As they walk, the poets begin to see the forest thinning. Here, there is no foliage of any kind. Planes of sand stretch out before them. The sand is unbearably hot. Dante peers closer and see hundreds of naked, condemned souls writhing and crying out in misery. He notices that those able to move about somewhat are less vocal than those buried in its depths:

“...naked souls beheld I many herds,
Who all were weeping very miserably,
And over them seemed set a law diverse.
Supine upon the ground some folk were lying;
And some were sitting all drawn up together,
And others went about continually.
Those who were going round were far the more,
And those were less who lay down to their torment,
But had their tongues more loosed to lamentation.”

Dante looks up and notices that not only does the burning ground torment the sinners, but so too do flakes of fire falling from the sky. The glowing ash sears whatever flesh is exposed. Those who are able to move can periodically avoid the falling flames, but those trapped in the sand, many buried from the waist down, suffer the ignition of the sand around them the flakes land. They frantically try to beat out the fire, and to Dante, it looks like a macabre dance.
Watching their horror, Dante notices one shade in particular. His eye is drawn to the enormous physical size of the former man as well as the giant’s loud complaints against God. Dante asks Virgil who the shade had been in life, but before Virgil can reply, the giant himself responds, but not with his name. Rather, he continues his vocal denouncement of God (whom he calls “Jove”) and, despite his predicament, claims that God will never will never be able to take revenge against him. The giant is committing the worst sin… blasphemy. He shouts to the travelers:

"Such as I was living, am I, dead.
If Jove should weary out his smith, from whom
He seized in anger the sharp thunderbolt,
Wherewith upon the last day I was smitten,
And if he wearied out by turns the others
In Mongibello at the swarthy forge,
Vociferating, 'Help, good Vulcan, help!'
Even as he did there at the fight of Phlegra,
And shot his bolts at me with all his might,
He would not have thereby a joyous vengeance."

Virgil knows who this blasphemous shade is. The man’s name is Capaneus; he had been one of the seven kings who fought against Thebes. The poets leave the howling giant and walk on; Virgil tells his charge to stay as close as possible to the outer edge of the sand, which is cooler.
The come to a small stream which is red with blood. It reminds Dante of a hot spring in Florence called the Bulicame, in which the local prostitutes bathed. Virgil tells him his stream is an offshoot the Phlegethon, the large river of blood that they crossed in Circle Six, just on the other side of the City of Dis.
Virgil, at Dante’s request, tells him more about the creation of the river. The origins of the rivers of Hell come from a weeping statue in the Crete. It faces Rome and its back is toward Egypt. The left leg is constructed of gold and silver, while the right leg is constructed of clay, each leg, perhaps, representing the strength of Rome and the decline of the Church, respectively. The entire statue is covered in cracks, from which leaks spring. The mysterious tears eventually seep through the earth and form the four rivers of Hell: the Acheron, the Styx, the Phlegethon, and the Cocytus.
Dante recalls a fifth river, the Lethe, and asks Virgil why he has not mentioned its presence in Hell. Virgil replies that the Lethe flows not in Hell proper, but in Purgatory, the plane that exists between Heaven and Hell. “Lethe” means forgetfulness. After having their sins purged in Purgatory, the sinners’ memories are wiped clean of their past transgressions so that they can enter Heaven with joy.
The poets move on, deeper into this torturous circle.