Jason Ritter, Lauren Graham Jason Ritter, Lauren Graham

The on-again, off-again relationship between Sarah (Lauren Graham) and Mark Cyr (Jason Ritter) on Parenthood has weathered its share of challenges: their age difference, the fact that her daughter was his student. But the couple is about to weather their biggest test yet now that Sarah's ex-husband, Seth, is back in town and headed to rehab.

"There's this happiness that he's finally taking care of himself but it's very stressful for what it means for Mark and Sarah," Graham tells TVGuide.com. "It's a very jarring thing to have Seth in town at all and I think still new for Sarah to be juggling any relationship issues. She's been on her own for a while."

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Tuesday's episode (10/9c, NBC) will be a big step forward for Seth (John Corbett) as he enters rehab to cure his long-standing addictions. But will it be a step backward for Sarah and Mark after finally reuniting earlier this season? Her support for her ex (both emotionally and financially, after she borrowed the money for Seth's treatment from Julia and Joel) may mean less time for Mark, but he's prepared to put up a fight for the woman he loves. "It's a difficult thing for him, but he knows that it's an important thing for Sarah, and if he were to try to put his foot down or draw a line in the sand, it would only push her away," Ritter says. "He's able to see on one hand that that is very selfless and beautiful of her, and on the other hand, he's nervous that maybe it will bring things up for her that will cause her to pull away from him. He's doing his best right now to stay in the relationship and also make some boundaries so that she knows that this is not easy for him."

Adds Graham: "He's no pushover."

Also coming to Sarah's rescue will be her two children, especially eldest daughter Amber (Mae Whitman), who lends a hand to protect her mom despite her rocky relationship with her dad. "I think this is the first time that they've started to see their mom happy. So even though at times the kids are at odds with Sarah, they kind of step up and help out and also push back at their dad and want to make sure that they're helping me do the right thing," Graham says. "It's funny because Sarah is one of the only characters who has kids this old and so we're really seeing them go out on their own and becoming adults."

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Also on Tuesday night's episode, Mark meets the rest of the Braverman clan.  "They meet Mark at a big party and give him a hard time, as they do," Graham says.

A family as big, and as involved in each other's personal lives, as the Bravermans may be intimidating, but Mark sounds ready (albeit a little nervous). "He's trying to make a good impression and hoping that everybody likes him, but at the same time, he has the knowledge that he and Sarah are really in love and that he hasn't done anything to be ashamed of," Ritter says — before Graham adds "yet" with a laugh.