What happens in Canto XVII of Dante's Inferno?

Canto XVII
At the end of Canto XVI, Virgil has thrown the rope that served as Dante’s belt down a waterfall. When the rope lands in a churning river below, its lasso encircled a monster. Then, Dante exclaims that some horror is caught in the loop but does not tell the reader what it is.
The beginning of Canto XVII reveals the beast. Virgil says:

"Behold the monster with the pointed tail,
Who cleaves the hills, and breaketh walls and weapons,
Behold him who infecteth all the world."

Virgil beckons that the beast raise itself out of the water. Dante sees just how hideous the monster is:

"The face was as the face of a just man,
Its semblance outwardly was so benign,
And of a serpent all the trunk beside.
Two paws it had, hairy unto the armpits;
The back, and breast, and both the sides it had
Depicted o'er with nooses and with shields.
At the bottom of the beast, it has a terrible tail:
His tail was wholly quivering in the void,
Contorting upwards the envenomed fork,
That in the guise of scorpion armed its point."

As Dante watches the monster hoist its enormous bulk halfway out of the water, its efforts remind Dante of Germans, and also beavers, an animal that typically leaves its tail in the water when it sits on the banks. (Germany, in Dante’s time, were plentiful in Germany.) There is another reason Dante selects the beaver for comparison to the monster. People erroneously believed (beavers do not eat fish) that beavers caught fish by using their tails to stir up the waters and then scooping them out with those wide, flat tails.
Virgil leads his reluctant charge towards the beast:

"Now perforce must turn aside
Our way a little, even to that beast
Malevolent, that yonder coucheth him."

Now very close indeed, Virgil spies a group of sinners sitting on some rocks nearby. He encourages Dante to go to them. Virgil will talk to the monster himself; Dante is stunned to learn that Virgil plans to ask the monster to carry them both on his back into Circle Eight:

"So that full
Experience of this round thou bear away,
Now go and see what their condition is.
There let thy conversation be concise;
Till thou returnest I will speak with him,
That he concede to us his stalwart shoulders."

Dante does as Virgil asks and hurries away to talk to the sinners on the rocks not far away; as he comes closer, he sees how they all flick their hands in agony, trying unsuccessfully to avoid the falling flames. Closer still, he does not recognize any individual face among their number. He does, however, recognize the pouches each wears around their neck. Some feature a blue lion on a gold background; several picture a white goose on a red background; finally, others display a pregnant sow on a white background. These symbols, Dante knows, are Florentine ones, used by wealthy families who made their fortunes through usury.
These shades are not interested in talking to Dante. In fact, one of their number tells Dante to leave them, for the place in which Dante currently stands is reserved for a man about to be condemned to the Seventh Circle of Hell. His name, the sinner says, is Vitaliano, another usurer, whose symbol on his neck pouch is three goats

“What dost thou in this moat?
Now get thee gone; and since thou'rt still alive,
Know that a neighbour of mine, Vitaliano,
Will have his seat here on my left-hand side.
A Paduan am I with these Florentines;
Full many a time they thunder in mine ears,
Exclaiming, 'Come the sovereign cavalier,
He who shall bring the satchel with three goats;'"

Dante heeds the shade’s request and returns to Virgil, where he finds his guide already mounted on the terrible back of Malevolent. Virgil bids Dante to climb on too, in front of him, and warns his charge to be cautious of the monster’s horrible tail:

"Now be both strong and bold.
Now we descend by stairways such as these;
Mount thou in front, for I will be midway,
So that the tail may have no power to harm thee."

Despite his nearly overwhelming fear, Dante does as Virgil bids; he climbs on the monster’s back. Virgil then calls the monster by name, Geyron, and tells it to go forward. The “novel burden” is Dante, the living visitor to Hell. Virgil commands:

“Now, Geryon, bestir thyself;
The circles large, and the descent be little;
Think of the novel burden which thou hast."

The monster complies, moving its enormous bulk backwards until it has enough room to stretch its hideous wings and take off. Its attempt reminds Dante of a boat backing away from a dock:

“Even as the little vessel shoves from shore,
Backward, still backward, so he thence withdrew;
And when he wholly felt himself afloat,
There where his breast had been he turned his tail,
And that extended like an eel he moved,
And with his paws drew to himself the air.”

As the beast gains altitude, Dante is reminded of the characters from Greek mythology, Phaethon and Icarus. Phaethon drove his father’s chariot too close to the son and perished. Icarus, in wings fashioned from feathers and wax, also rose too close to the sun and died. Clearly, Dante is terrified.
A brief look downward only increases Dante’s fear. But his time in Hell thus far has strengthened his resolve and his fortitude; he does not pass out but clings even more tightly to Geyron’s scaly back.
Geyron begins a descent in slow circles; the movements remind Dante of a falcon. FInally, Geyron touches ground. His passengers scramble off his back and Geyron sails away, leaving the poets alone in Hell’s Eighth Circle:

"Even thus did Geryon place us on the bottom,
Close to the bases of the rough-hewn rock,
And being disencumbered of our persons,
He sped away as arrow from the string."