The merger of Seattle Grace and Mercy West isn't the only problem the residents of Seattle Grace are having when the show returns tonight with "Tainted Obligation."

Thatcher Grey (Jeff Perry) returns and throws the Grey sisters into turmoil. Lexie (Chyler Leigh) fights to save her dad's life. Meredith (Ellen Pompeo) doesn't care.

In an interview with TV Guide, Leigh says that Lexie's budding relationship with Mark (Eric Dane) will have a wrench thrown into it. What else could go wrong? Plenty.

Here's what Chyler Leigh has to say about tonight's episode and beyond ...

On Lexie's daddy issues: "Thatcher comes into the hospital and is seemingly drunk. There's this whole question where I'm saying, 'No he's not drunk, something else is going wrong.' Meredith sees him come into the hospital and goes, 'Oh, he's up to his regular antics again.'"

"I have to convince her there's something else going on. You'll find out that he's got some really intense liver issues going on [and] he has to have a transplant."

On Lexie's sister issues: "Meredith is questioning whether or not she should even care [about Thatcher], and whether or not she should feel bad that she doesn't care. Lexie got this whole and healed kind of a guy and had this really great upbringing. Meredith had an entirely different upbringing of neglect and abuse. It is like you're looking at two separate people."

"It stirs up some pretty big issues between the sisters and ultimately works out really well. Ultimately Meredith helps her, but not in the way that everyone is expecting."

On the merger: "In the first few seasons, the doctors were really fierce with one another in trying to get cases and surgeries. That comes back this season. There's an influx of doctors coming in [that] sends everybody into a panic, scrambling to make sure their job is secure. There are four doctors that come into the hospital that are some seriously fierce competition."

On comings and goings: "Nobody is sure who is going to stick around. The people who you think are safe, might not be. The people that you think [are] totally going to be the ones to go, might not. It really throws a lot up in the air. Lexie, who tends to be a little more insecure about her abilities, she gets thrown easily with competition."

On Lexie and Mark: "Their relationship right now is pretty stable. They're making good progress, but there is definitely something that comes up soon that throws a wrench in the whole thing. They're coming up on some rough roads."

"As far as I've been told, it's pretty intense and I'm just hoping that they do [last]."

On Meredith and Izzie: "Meredith is still in all the episodes. They had to get pretty creative on how to give Ellen an out. With Katherine's Izzie being gone, it throws a lot on the plate for Alex and how he has to deal with her being out. It certainly ups the drama factor."

"You'll see soon enough."

Steve Marsi is the Managing Editor of TV Fanatic. Follow him on Google+ or email him here.