Dorian is (literally) a model of youth and beauty (Chapters 1-2)
Dorian's nature is unspoiled and his exquisite outer beauty mirrors the pure inner beauty of his soul. He's as innocent as the day he was born… until a certain young Lord enters the picture.
Trouble in paradise—art conflicts with life in Dorian's relationship with Sibyl Vane (Chapters 3-10)
To cut a long story short, Dorian idealistically falls in love with Sibyl, and, upon realizing the fact that she doesn't live up to his expectations, he dumps her. She kills herself, and instead of mourning her and learning a lesson, Dorian reads the yellow book, listens to Lord Henry, and gets over the whole thing.
"Poisoned by a book" (Chapters 10-11)
We're not exactly sure what Dorian's up to over the next decade or so. He's deeply influenced by the yellow book, and consequently changes his mode of living. Though things look peachy keen on the surface, rumors start to emerge about Dorian's secret, evil deeds. We don't know any details, but it seems like our hero has gone completely over to the dark side.
Dorian is now all evil, all the time (Chapters 12-15)
All bets are off—Dorian seems to have lost all vestiges of his former self. He doesn't even have any feelings left for Basil, formerly his best friend; in fact, even after he kills Basil in a fit of passion, he pretty much feels like B. brought it upon himself.
Like Lord Henry, Dorian seems mostly to be filled with a vague sense of pity and contempt for everyone else. To top it all off, he blackmails another ex-friend, Alan Campbell, into covering for his crime.
Fear and self-loathing in London, then the countryside (Chapter 16-17)
Dorian is understandably shaken by Basil's murder, but not for reasons we'd expect; rather, he's terrified that he'll get caught. To make matters worse, he discovers that James Vane (brother of Sibyl) is back in town and on the murderous prowl for him. Dorian is wracked with fear of death, first in London, then when James follows him to his country home at Selby.
Life lessons from Oscar Wilde—if you're stalking someone during a hunt, don't hide out in the line of fire (Chapter 18)
It seems as though everything has worked out for old Dorian Gray—James Vane is accidentally killed at Selby, which means that there's nobody out looking for him. He feels a profound sense of relief, and wonders if he should change his ways after all.
The inevitable happens (Chapter 20)
After thinking that he should turn over a new life, Dorian basically says, "Screw it!" and decides to keep on going the way he's been going. He loves being evil, and realizes that even the thought of becoming good makes him a hypocrite, a new sin to add to his catalog. However, morality triumphs, and Dorian finally gets his comeuppance—by trying to destroy his portrait (read: his soul), he kills himself.