Mark Wilding, the head writer on "Invasion," has a lot of explaining to do.

Where the heck did Izzie go? Will she be coming back? And what is with these people coming in and tearing down the foundation of the Seattle Grace we know and love?

Below are some excerpts from Mark's column, with a link at the bottom to her full entry. It's always interesting to get a fresh take from the people who put it together:

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Izzie is gone. Fired by the Chief who, these days, is looking an excuse to fire ANYONE. Maybe she went back home. Or to visit some friends. Or just took off for points unknown.

But yes, Izzie Stevens, third year resident and surgical rising star, no longer works at Seattle Grace/Mercy West. The hospital, new name and all, is in her rear view mirror.

George’s death, her stage four cancer, coming back to work too soon – it all proved too much – and now she’s off to God knows where. More on Izzie and broken-hearted Alex later.

Reed, Charles, April and Jackson. The Mercy West crowd. Just as insecure and uncertain about their new situation as our people. And our residents are understandably nervous because, like invaders of any kind, these MW guys are a threat. They’re smart, they fight dirty.

The new actors – their real names are Nora, Robert, Jesse and Sarah - were a blast to work with. I probably should have known all their credits and what shows they’ve been on before we started shooting but I didn’t. And none of that mattered because they were all so strong.

Lexie and April. We got to see a new side of Lexie Grey. One who can fight dirty and be a mean if she had to be. That is until the passive-aggressive April herself is undone when she finds out Lexie has read her notebook with all her little self-help homilies. 

Izzie was looking over her shoulder from the get-go.

Charles and Izzie. Izzie thinks she’s found someone who maybe just maybe, reminds her a little of George. And it goes to Izzie’s trusting instincts and her need to fill the void left by George that she quickly gloms onto a sympathetic soul like Charles.

But, as the Chief tells her later, it’s a different era. The old rules don’t apply – be it with friendships or with jobs. A lesson that Izzie learns not just once in this episode, but twice.

Jackson and Cristina. I especially loved Cristina Yang finally deciding to hold up the white flag of surrender. Throwing in the surgical towel. And as for Sandra Oh - in the scene when she’s crying with Meredith (and pining for Burke), well, my God, she was superb.

Alex and Reed. Reed is not a big person. Still, as Alex finds out, she can flat out play this game. As much as I loved their competitive dynamic, each trying to outmaneuver the other one, I also liked showing our doctors as less than compassionate with a patient.

Back to Izzie. When you tell the Chief of Surgery that you NEED this job, that you’ve got nothing else in your life, DESPITE the fact you’re married, you are in a very bad state indeed.

I understand the part about sneaking off without telling anyone after you’ve been fired but to just leave your husband a note – that’s absolutely brutal. No wonder Alex has issues.

Finally, I want to talk about Callie and Arizona. It was Hector Elizondo’s idea to use the cards. He didn’t think his character would know that stuff of the top of his head, which is how I originally wrote it. And he was right. And it worked beautifully.

And that final scene when he and Callie are outside the hospital and he’s asking her if she’ll still get married, still have kids. I really, really liked that scene. 

Follow the link for the entirety of Mark's blog ...

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