Fans of writer/producer Joss Whedon aren't going to have to wait until Halloween for a sweet treat, since the night before – on Friday – Summer Glau begins her recurring role on Whedon's science-fiction drama “Dollhouse.”

Glau comes to the Fox show after appearing in two previous Whedon series – The WB's “Angel” and Fox's “Firefly” – and starring in Fox's “Terminator: The Sarah Connor Chronicles,” which shared Fridays with “Dollhouse” last season.

Now in its second season, “Dollhouse” is the saga of a powerful and mysterious corporation that has the technology to wipe and then reprogram the minds of volunteers. These “Actives” then play client-requested roles in very expensive “engagements.”

They serve a five-year term, then have their original personalities returned and are set free. But at the sleek, Asian-themed Los Angeles facility – called the “Dollhouse” – an Active named Echo (Eliza Dushku), who used to be a politically active girl named Caroline, doesn't quite keep with the program.

Now things are starting to go seriously haywire in Rossum Corp.'s carefully controlled world.

In “The Public Eye,” the first part of a two-part story that concludes with “The Left Hand” on Nov. 6, Glau creates the character of Bennett Halverson, who arrives at the L.A. Dollhouse from the one in Washington, D.C. She works there as a technical whiz in charge of wiping and reprogramming the Actives.

“I see Bennett as a sensitive, creative girl,” Glau says, “who spends all of her time alone in a world of her own. She is a mad scientist who is searching for happiness.

“This is a challenging role in many ways. The most challenging thing is the dead arm!”

Apparently, Bennett and Echo's alter ego have a history. “She has a vendetta against Echo,” Glau says. “She knew her back when she was Caroline, and there was this horrible accident, and she has been waiting to reunite with her for quite some time – waiting to get my revenge.

“I'm making her as creepy as it gets, but I asked Joss, ‘Can I still be sexy?'”

“The character that she plays on ‘Dollhouse,'” Whedon says, “is very sweet and calm and elegant, kind of shy, kind of adorable, possibly evil, but that doesn't matter.

“She's a little bit damaged, because that's what I write. She's played her share of crazies, and I didn't want that. I just wanted someone who was a little off-kilter and yet enormously sympathetic.”