In 1997 a demon by the name of Whistler, happened into Angel's path. By this time Angel was feeling the effects of his soul almost to the point of insanity and was living off of rats.
Whistler offered Angel the chance to be someone. To do something with his unlife. He took Angel to L.A. and pointed out Buffy Summers from afar. Angel was there, in the shadows when Buffy was called to be The Chosen One, The Slayer.
Angel didn't meet Buffy at this time. He followed her to the Hellmouth in Sunnydale and chose the impending Armageddon, "The Harvest", as the time to make his presence know to her. Warning her of the threat.
Angel became something of a phantom of the night, or as Buffy called him "Cryptic-guy". He would show up when she was patrolling, or reaserching "The Harvest", offering up helpful information and then disappear again into the night.
Buffy quickly became infatuated with Angel and they developed a friendship. Angel revealled his vampiric nature to her, explaining how he was different from other vampires and she didn't stake him.
Shortly after this Darla re-entered Angel's life, in the service of her Sire, The Master, who was responsible for the coming "Harvest". Darla tried to come between Buffy & Angel, and in attempt to get back at him managed to convinse Buffy that he had attacked her mother, Joyce.
Buffy hunted Angel, but found herself in danger of being killed by Darla. Forced to chose between his old love and new, Angel staked Darla. Shortly afterwards Buffy succeeded in killing the Master and stopping the Hellmouth from being opened.
From this point on Angel & Buffy became an item, fighting side-by-side and falling in love along the way. On Buffy's Seventeenth birthday they made love for the first time and Angel had his first moment of True happiness in Centuries.
This moment was to change their relationship forever. Angel lost his soul, and Angelus returned with a vengance.
At this point, a young woman by the name of Jenny Calendar was working at Sunnydale High. Jenny was a decendant of the Clan that had originally cursed Angel and was the only one able to return his soul.
Angelus stalked and tormented Buffy and her friends and eventually killed Jenny as she was about to return his soul. In an act of extreme evilness, Angelus then left her body in the bed of Buffy's watcher Giles, who had been falling in love with Jenny.
With the help of his Childe Drusilla and her Childe Spike, Angelus developed a plan to end the world sending them all to Hell. Willow was working on a spell to restore his soul but by the time she had done so Angelus had already managed to open the gates to Hell. Angel's soul was returned just in time for Buffy to have to run a sword through him, sending him to Hell and closing the gates. It was the only way to save the world.