With the departure of original star Shannen Doherty -- and the death of her character, telekinetic witch Prue Halliwell -- at the end of last season, "Charmed" enters its fourth year on Thursday, Oct. 4 at 8 p.m. ET, with someone lost and someone found.

Up until now, the series has focused on three San Francisco sisters -- Prue (Doherty), Piper (Holly Marie Combs) and Phoebe (Alyssa Milano) -- who discovered they were The Charmed Ones, good witches destined to use their magical powers to fight evil.

As last season ended, Piper was finally married to Leo (Brian Krause), a "White Lighter" (mystical guardians who help good witches); while Phoebe was struggling with the realization that her great love, Cole (Julian McMahon), has spent most of his existence as a murderous demon, hidden in human form.

As the season opens with a two-hour episode, "Charmed Again," written by executive producer Brad Kern, the remaining two sisters are struggling to cope with the loss of Prue, who had long been the series' driving force. Also coming into their lives is a new element -- a long-hidden half-sister, Paige Matthews, played by independent-film actress Rose McGowan. The girl is the product of an illicit affair between the sisters' witch mother and her own White Lighter, and was hidden away to be raised by an adoptive family.

In an interview earlier this summer, shortly after McGowan was cast, Kern describes Paige thus, "From Paige's point of view," says Kern, "you go to bed one night, and you think you're a single child, and no one understands you. Then you wake up the next morning, and you find out you've got sisters who are witches, who've got supernatural powers, who have a destiny that you must now share and embrace, and you must learn your new powers."

"It won't be a coincidence that she was named with a name that starts with a 'P,' like her sisters."

According to Kern, Paige's newly emerged power resembles Prue's ability to move objects with her mind, but because her father was a White Lighter, she will have other abilities as well.

"When we meet her," says Kern, "she will have been searching for many years, both for her birth parents, but also for an answer to that feeling that she's had inside of her that there's something special, there's something else that nobody has yet been able to understand and explain."

"We hope to tap into that. It's also organic to the character."

In an interview conducted during the filming of the fourth episode (with seven scripts total written), Kern elaborates, "We're still feeling each other out as far as what the show is with Rose. We're recreating the show in many ways. As you know, it's mostly the same show, but when you bring in a new character, that changes."

"Just by watching the three of them begin to work together, we've made some adjustments, but it seems to be working out very well. For the first third of the season, she's the outsider, trying to get into the charmed circle. That's where we're at. As we finish the first third of the season, we finally bring them together, as far as moving in together."

So the wholehearted acceptance of Paige doesn't happen immediately? "God, no," says Kern. "It wouldn't be honest. That wouldn't be authentic to the loss of the sister, just to suddenly go to a new sister. We're trying to play it as authentically as possible. At the same time, Rose's character is a very ambitious and excitable character who is new to the Craft and enjoys it so much that it actually propels all of them to deal with the present and not dwell in the past."

"That accelerates Phoebe's and Piper's mourning period, so they can get on with their lives. But it's going to take a third of the season to even begin to get on with their lives."

"The challenge is, 'Charmed' is a fun show, and we don't want that mourning to drag that down. At the same time, we don't want to ignore the mourning process, so it's been a balancing act. Of course, when demons are attacking every week, it tends to keep you busy."

Also on hand for the season premiere is Jordan Bridges, the twentysomething son of Beau Bridges. He plays Shane, who's been dating Paige for about a month. "He's her boyfriend," says Kern. "Then he gets possessed, he's evil. He plays it well, too. There's a possibility he'll be back. He's just in the opener right now."

What sort of a guy is Shane? "He ends up helping us understand who Paige is and where she's coming from. He's a good guy, and naturally, when you're a good guy, bad things happen to you on 'Charmed.' Paige will see firsthand what it's like to be a witch. The impact that it has on her own personal life will help her better understand what Phoebe and Piper have had to deal with for the past three years. It strikes home."

The changes also mean a whole new role for former middle-child Piper. "Holly's doing a great job of taking that over. She's very affected emotionally by the loss of Prue in the first couple of episode, but she learns, despite herself, to rally and take over the older-sister role, and she may not like it."

"She's always been the most reluctant of the witches to embrace the Craft, but at the same time, she's a pragmatist. At the end of the day, she knows she has to take over the mantle."

"Holly's doing a terrific job of going from the Solomon-like middle sister to the kick-butt, 'I'm in charge, and this is what we're doing' sister."