<i>Hawaii Five-0</i> Hawaii Five-0

Hawaii Five-0's Steve McGarrett just can't catch a break.

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After putting his nemesis Wo Fat (Mark Dacascos

) behind bars at the end of Season 2, McGarrett (Alex O'Loughlin) was forced to contend with another huge surprise: His believed-dead mom is not only alive, but she's also the mysterious Shelburne. But don't expect a warm and fuzzy reunion when Steve and Doris (guest star Christine Lahti) come face-to-face in Monday's premiere (10/9c, CBS)."She has a lot of explaining to do," executive producer Peter Lenkov tells TVGuide.com. "She's been gone for 20 years. She faked her death, and she believes she did it to protect her family — she believes she made this ultimate sacrifice. The thing is: Do you believe her? That's what McGarrett is going to wrestle with for the first few episodes at least."Lahti says her character knows that convincing her son to trust her again is an uphill battle. "She hopes to be able to repair [the relationship], but does she figure that it's not repairable? Yes," the Emmy and Oscar-winning actress says. "She does have a lot of guilt about how much hurt she's caused her family, but I don't think she has any regrets. I think if she had to do it over again she would do the same thing.

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"One thing that can't be questioned about Doris is her love for her children," Lahti continues. "That has not gone away or diminished. I think, if anything, she would love them to understand why she did what she did and she is just hoping for their forgiveness. But she knows that might be too much."As McGarrett is forced to re-evaluate what he believed to be true, Lahti says viewers will be treated to a new, more emotional side of Five-0's steely leader. "His mode, his go-to place is that tough guy, but in the scenes I've done with him, there's been so much vulnerability and flashes of so much deep pain," Lahti says. "There is a lot of pain there, even though he can't access it as readily as most people. But it's there underneath. Underneath the rage and the betrayal and the anger is a lot of hurt and love for his mom."Unfortunately for Doris, cracking through McGarrett's icy layers is only the beginning. Danny (Scott Caan), Catherine (Michelle Borth) and the entire team will be looking to protect Steve from any further emotional damage. "You want to believe that she's good, but at the end of the day, there's a lot of gray in her character and a lot of questions," Lenkov says. "She's still a person who keeps secrets. Our team is very open with each other, and for her to be keeping secrets, I think they're going to be wary of her."

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Adds Lahti: "On the one hand she's very happy that he has a family like these people. On the other hand, she feels it's this tremendous wall she has to climb... and it may be impenetrable. She probably feels a little jealousy, [but] she's grateful that her son has such close, loyal friendships."Whether or not McGarrett believes a word out of his mother's mouth, he'll still have to protect her.  "Now that she's surfaced, it's a little bit like opening Pandora's box," Lenkov says. "She's someone who other people have been looking for. She was an operative, someone who was clearly an asset. So there's going to be other people coming out of the woodwork."That includes Wo Fat, who, naturally, won't stay in prison very long. But does Doris actually need protecting? "She has been under CIA protection for 20 years, but my assumption is that she hasn't been a schoolteacher," Lahti says with a laugh. "I get to play more of a kick-ass kind of character than I'm used to and that's really been fun. Apparently the [next episode] coming down the pike, I go all Jack Bauer!" Adds Lenkov: "She can definitely take care of herself. She's pretty resourceful — she's strong and she's tough. She killed Wo Fat's father. All those stories about Shelburne are true."

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Perhaps most importantly, Lenkov says McGarrett's interactions with his mother will deepen the show's mythology and give McGarrett more answers about his past. But Lahti says you'll be waiting all season to figure them out. "She holds a lot clues to the mysteries around Steve's life and he's going to be really gratified to finally find out," Lahti says. "There's a lot of stuff that I actually know that even Alex doesn't know about his character, but I've been sworn to secrecy. The e-mail actually said, 'This e-mail will self-destruct in five minutes!'"Hawaii Five-0 premieres Monday at 10/9c on CBS.