What happens in Canto VIII of Dante's Inferno?

Canto VIII
Dante and Virgil gladly leave behind the Wrathful and the Sullen in Circle Five. As they walk on, Dante realizes that a light has been guiding them. The pair descend further into Hell:

We to the foot of that high tower had come,
Our eyes went upward to the summit of it,
By reason of two flamelets we saw placed there,
And from afar another answer them,
So far, that hardly could the eye attain it.

The poets have come to the dividing line between upper hell (Circles One through Five) and lower hell, (Circles Six through Nine). The worst offenders are housed here. In Circle Six, prior to entering the City of Dis, are the heretics, those who oppose Catholic doctrine.
Unwilling or unable to turn back, the pair proceed. In the distance, Dante observes a flickering light, unlike the ones glowing from the faraway tower. He asks his guide where it is coming from. Virgil implores his charge to look harder. Finally the ferriman comes into view. It is Phlegyas, son of the Greek god Ares, now tasked with taking souls from one side of the stinking Styx to the other. After a few harsh words are spoken between Virgil and Phlegyas, the oarsman delivers his charges to the opposite shore.
While rowing across, Dante observes condemned souls in the fetid waters. Gone is his usual modicum of compassion for the sinners. Instead, when the shade refuses to give his name, Dante curses him and Virgil shoves the sinner back into the slime.
Dante soon discovers who this heretofore nameless soul is, and why his guide is so virulent in his repulsion. This disgusting soul is Philippo Argenti, a Florentine and a member of the Black Guelph party. Allegedly, Argenti opposed Dante’s return from exile. The poet punishes his oppressor by eternally immersing the man in the River Styx, where he is accosted not only by other sinners, but turns violently upon himself as well. As the travelers are pulling away, Dante watches as the terrifying hoard attacks:

They all were shouting, "At Philippo Argenti!"
And that exasperate spirit Florentine
Turned round upon himself with his own teeth.

Glad to leave the horror behind, the poets push on. Dante remarks on the red glow in the distance. Virgil confirms that this is the City of Dis, and that its gates are heavily fortified.