This is off the top of my head, so I hope others will correct my often faulty memory where applicable.
In (very) brief...
We pick up some weeks after Don't Fade Away. L.A. is in Hell, Angel is human and has adopted the dragon he was going to kill when last we saw him. Gunn is a Vamp. Spike and Illyira have set themselves up as Lords of Beverly Hills(?) and Lorne has done the same in Silverdale.
Gunn has a vamp posse and a plan to kill Angel and thereby save L.A. (if that doesn't make sense, realize that Vamp!Gunn is being manipulated by W&H.)
Ghost Wesley helps Angel at the behest of W&H. Illyria is having Fred flashbacks and freaks when she see Wes.
Angel challenges the various Lords of Hell, and with the help of the Fang-Gang defeats them.
In final battle with Gunn, Angel the human is mortally wounded and is met by Cordelia, who is to escort him to the afterlife. However, in order to save his son, he returns to life.
Back in L.A., Angel and Wes realize that if Angel dies, the plans of W&H will be foiled as they want Angel to suffer eternally. Rather than let Angel die, W&H reset everything to the moment that L.A. went to Hell (no jokes). This time Angel is able to save Gunn before he dies and is vamped.
In an unusual twist on the contrived "re-set button" ending, everyone-- heroes, villians, dragons and innocent bystanders alike all remember what happened and Angel is looked upon as a hero.
In what might be something of an "unofficial" crossover with the Buffy comics, the existance of Vamps and Slayers becomes public knowledge at roughly the same time in both books.
Thanks. A few follow up questions:
Does Connor die? I'd heard somewhere that he dies.
What roles do Gwen Raiden, Kate Lockley, Nina, & Groo play in the story?
What exactly is Wolfram & Hart's plan?
What's the deal with the whole Fred/Illyria thing?