For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Dante is rescued by Virgil within the dark wood. They enter Hell. In the first five circles, Dante shows an excess of compassion for the incontinent sinners. Towards the end, Dante rebukes Filippo Argenti, much to Virgil’s delight. At the city of Dis, Virgil’s "persuasive word" fails for the first time and the pilgrims must be rescued by a heavenly messenger.
Act II
Dante and Virgil journey through the circles of heresy and violence. Virgil explains the structure of Hell and the three categories of sin. Dante shows a great deal of sympathy for the sinners—especially Brunetto Latini and Pier della Vigna. Virgil explains the origin of the five rivers of the Underworld. This section ends with Virgil summoning Geryon from the depths and scaring the hell out of Dante when they ride him down into the eighth circle.
Act III
Dante and Virgil move through the Eighth and Ninth circles. Throughout, Dante shows less and less sympathy for the fraudulent sinners, especially Pope Nicholas III, Vanni Fucci, the barrators, and Fra Alberigo. Virgil convinces a giant to take them into the ninth circle. They witness Lucifer, who turns out to be anticlimactic. And finally, they return to the surface of the earth, where a new dawn awaits them.