'Firefly Lane' Reveals What Tore Kate and Tully Apart: Sarah Chalke on Their Breaking Point (Exclusive)

Sarah Chalke breaks down the events that led the BFFs to no longer be on speaking terms and whether a reconciliation is in the cards.

Spoiler alert! Do not proceed if you have not watched Part 1 of Netflix's final season of Firefly Lane.

Firefly Lane finally answered the show's biggest question: What took place between Kate and Tully that was so serious the BFFs were no longer on speaking terms? The revelation that the duo was estranged, revealed in a flash-forward in the freshman finale at the funeral of Kate's father, stunned viewers. As Firefly Lane star Sarah Chalke previously teased, "You’re going to get all of the answers and it's crazy. I don’t think it’s going to be what you thought it was going to be."

Sure enough, she was right. In the seventh episode of Part 1 of Firefly Lane season 2, viewers got the answer they've been waiting for with regard to the circumstances that led to Kate and Tully's estrangement -- and it was a slight departure from the Kristin Hannah novel on which the series is based. Though both involve Kate's teenage daughter, Marah, the series had Tully babysitting Marah at her apartment after Kate and Johnny decide to celebrate their reconciliation. (In the novel, Kate and Tully's friendship deteriorates after Kate learns that she and Marah have been brought on as guests for a segment on Tully's show about domineering mothers. Kate immediately cuts ties with Tully in response.)

That evening, Marah gets a text message from Ashley, a girl she has a crush on, asking her out to the movies and Tully decides to let her go out, even though Kate and Johnny advised her not to since Marah was grounded. After Marah leaves with her friend, Tully bumps into her ex, Danny, and he later reveals he's dating someone. After that news, Tully has a few glasses of wine before she gets a frantic call from Marah, who's suddenly in trouble.

Tully learns that Marah and Ashley actually went to a frat party after the movies, and Ashley ditched her to leave with her ex-boyfriend when they arrived. Abandoned and without a ride, Marah shares in the frantic call that she met an older guy at the party who offered her beer and was uncomfortably close to her. Desperate to take herself out of the situation, Marah hid in a small pantry at the party, where she called Tully from.

Worried for Marah's safety and reminded of the time she was raped in high school, Tully rushes to get Marah and successfully rescues her from the party. However, Tully -- who had wine right before getting behind the wheel -- and Marah, while crossing the intersection at a green light on their way home, are struck by another car who runs the red light. As a result of the accident, both suffer serious injuries, and though Tully technically wasn't at fault for the crash, she's arrested for drunk driving.

After several failed attempts to get Kate to talk to her, Tully -- who's lost her new talk show due to the ordeal -- finally succeeds in a meeting when Marah sends a text from Kate's phone inviting her to come over in episode 8. Though Kate's unhappy that Marah pulled the stunt to get the two of them to talk, she hears Tully out, who repeatedly apologizes for her actions that evening -- a big moment as it's difficult for her to say sorry. Though the BFFs clearly still miss each other, as they acknowledge during their heated conversation, the accident is enough for Kate not to ever speak to Tully again and not to offer forgiveness. 

"Going into it I didn’t know. I thought so much about, 'OK, what’s it going to be? What could this possibly be that Tully did that is big enough to break this unbelievably unbreakable bond of this friendship that has gotten both of them through their whole lives?' That’s the most important person to each other so what could it possibly be that would do that, but also has the possibility of them coming back together in the future," Firefly Lane star Sarah Chalke told ET's Deidre Behar in reacting to the reveal. "I thought the way that Maggie wove that in was so thoughtful."

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The actress explained why she believes Tully's apology for that night wasn't enough to mend bridges for Kate.

"We talked a lot about it and I love the fact that you really see both sides. I think it’s not enough for Kate. I think it’s a big deal and Kate wants to forgive her so badly because obviously she wants her back in her life so badly," Chalke offered. "But it was crossing a line. It was too much. It was the fact that she had been so clear: 'Marah's' grounded. [It] was the fact that Tully had been drinking, the fact that there were still so many other options on the table to her of calling a cab, calling Kate. Call anyone. None of those things should have played out and the fact Marah was actually quite hurt and she was her baby."

Chalke acknowledged that Kate reached her breaking point at that moment. "It was," she theorized in regard to Kate's reaction. "On the flip side of it, Tully because of her past and because everything that happened in season 1 with Pat [who sexually assaulted her during a high school party], it just triggers her the second she hears Marah's voice on the phone. And you know that she’s just like, 'Drop everything. Let's go. I just have to go to her right now,' and she forgets about anything and everything else."

"You really feel for that and you really see that side of it too, so what I loved about the decision was that it was something that you could really see both sides of and relate to both of them and understand both of them and really see where both of them were coming from. So not black and white," Chalke added.

Asked what it would take for the best friends to get to a point where they're at a good place again, the actress danced around the question, hinting that that will all come to pass in Part 2 when the final seven episodes drop next year. 

"Well, I can't tell you that!" Chalke teased. "Throughout their lives and through season 1, they had little hiccups, right? They fought but they always came back together. Obviously there was never anything like this that was this big, so." 

Chalke wouldn't offer any hints as to whether things will thaw between the duo, especially with Kate dealing with the news of her devastating breast cancer diagnosis.

"I don't know," she played coy, sharing later that they've completed filming on all of season 2. "I love Part 2. There's a lot of challenging stuff we filmed, there's a lot of really fun stuff. The show always gets you outside of your comfort zone; you're never safe inside your little box."

Part 1 of Firefly Lane's final season is streaming now on Netflix.

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