Los Angeles ? Joss Whedon’s sci-fi/Western series “Firefly,” set to air Fridays this fall on Fox, has had its ups and downs so far, what with a last-minute pickup and revisions to what will be the opening episode, but actor Sean Maher is still happy to be there.

Well-known to Fox viewers for his guest appearances on “Party of Five,” starring roles as a rookie cop in “Ryan Caulfield: Year One” and a New York stockbroker in “The $treet,” Maher had to take his new role pretty much on faith. A lot of that faith had to do with Whedon, who previously created “Buffy the Vampire Slayer” and co-created its spin-off, “Angel.”

The dark-haired Maher who looks slimmer and more refined in person than he did in his police uniform on “Ryan Caulfield” plays Simon Tam, the medical officer on the transport spaceship Serenity, whose crew struggles to make ends meet in the wake of a universal civil war, 400 years in the future.

“I’m not actually part of the crew,” says Maher. “I’m a doctor. It’s so hard to simplify anything. There are definitely two major ruling planets, and I’m from one of those planets. If you’re from one of those planets, you’re more privileged. You have a lot of money.”

“So I’m a doctor, a trauma surgeon. My sister is this prodigy, this amazing genius, so we sent her away to this government-sponsored academy that we think is just an amazing school. Then it turns out that she is part of this government test, and they screwed with her brain.”

Texan Summer Glau who previously appeared as a prima ballerina in “Waiting in the Wings,” episode of “Angel” directed by Whedon has been cast as Simon’s sister, River. She’s described in the press materials as “borderline psychic.”

There’s also the spaceship. “(The sets are) phenomenal,” Maher says. “They built one-half of the ship on one sound stage and another half of the ship on another sound stage. I was like a kid in a candy shop.”