Angel tells the story of a vampire with a soul, who teams up with a group of people to help the helpless in present-day Los Angeles. It is a spinoff of Buffy the Vampire Slayer.

Angel (David Boreanaz), the vampire with a soul, has left Sunnydale, CA and moved to Los Angeles after a breakup with Buffy Summers (Sarah Michelle Gellar). He opens a detective agency, Angel Investigations, whose mission is "to help the helpless", with the support of Cordelia Chase (Charisma Carpenter), also from Sunnydale, and Allen Doyle (Glenn Quinn), a half-demon who can sense trouble. Over the next several seasons, many new characters join the investigation team, several die, and their mission evolves.

While Buffy the Vampire Slayer had a generally optimistic tone and a great deal of humor, Angel was significantly more downbeat. Its core theme was doing things to help others, even in the face of inevitable defeat. Angel also focused on how different people, even those who are theoretically on the same side, may have vastly different ideas of what is the right thing to do. Characters who are "good" often wind up in opposition to Angel's mission, and characters originally termed "evil" join Angel in doing the right thing, for their own reasons.