Expectations are often muted for spinoffs. After all, any piece of the whole by definition must be less than that from which it has come. For every successful spinoff such as “Frasier,” there’s a dozen shows like “AfterMASH,” bad concepts which assemble a handful of supporting players and try to succeed on name recognition alone.

“Angel,” debuting at 9 tonight on WB, has the potential to be another “AfterMASH,” gathering a couple of complementary characters from the smash “Buffy the Vampire Slayer” and transplanting them to Los Angeles. And yet, “Angel” works on its own minimalist level.

For those not initiated to “Buffy,” Angel (played by David Boreanaz) is a 200-year-old vampire whose soul has been restored by a Gypsy curse. As a result, he feels tremendous guilt for all those he has killed through the years. Yet if he ever knows “perfect happiness,” he reverts to his previous evil self. That’s why his romance with Buffy could never work out, so he moved to L.A. to keep from endangering her.

Also relocating to the City of Angels is the exceptionally self-possessed Cordelia Chase (Charisma Carpenter), who wants to become a star, with very little success so far. She and Angel end up teaming up in a mutually beneficial platonic relationship.

Guiding Angel is the demonic Irish rogue Doyle (Glenn Quinn, “Roseanne”), who receives visions and sends Angel on missions from the ethereal “The Powers That Be.” To atone for his many wrongs, Angel must help troubled souls in need (think of it as “Touched By a Vampire”).

“Angel” is considerably darker in tone than “Buffy,” but still, humor permeates the series, thanks to executive producers Joss Whedon and David Greenwalt. Whether he’s jumping into the wrong vintage muscle car or being solicited by an actor’s manager, it soon becomes clear that Angel isn’t in “Buffy” setting Sunnydale anymore.

“Angel,” almost a detective-action series, also promises to be more episodic than “Buffy,” but then the debut hints at dark supernatural undercurrents in L.A. as well, which is likely to lead to a continuing story line. “Angel” doesn’t possess the quality casting of its lead-in, “Buffy,” but it still stands tall on its own terms and succeeds.