In part one of a Grey's Anatomy / Private Practice crossover, Derek's working on Tom Kates, Seattle's star pro quarterback who suffered head and neck trauma after an ugly hit he took on the field.

It turns out he's got a concussion and needs to have his spleen removed, which will bench him until next season. He lights up when he sees his wife and baby son.

Then he has a panic attack when Derek talks about him playing again. He confesses he doesn't want to play any more. He used to be able to fight through his fear, but now that he's a father he realizes what the game can do to him.

But he's two years into his pro career - he can't quit now.

Derek and Callie take a look at Tom's knees, which are legitimately shot, and Derek realizes that they've found a way out for him - a knee replacement surgery that would effectively end his career and would not make it look like he walked away.

Alex and Reed (who's messing with Alex and seems to have led him and others to believe she'd trade sex for surgeries) assist, and Callie reminds them that it's not a game, it's the man's career.

In the midst of this, Derek sniffs out a problem with the Chief. He's helping Bailey with the follow-up on the gall bladder patient whose surgery the Chief messed up a couple of months ago, and Derek picks up on the fact that a mistake was made.

The Chief hasn't been in surgery since.

He basically bullies the Chief into taking Tom's spleen surgery, then confronts Bailey with his suspicions when he doesn't show up. Meanwhile, the Chief, in his role as mentor to Meredith, has been watching old films of her mother's surgeries with her.

Meredith ultimately confesses to Derek that the Chief's drinking again.

Elsewhere, Teddy hands Cristina a solo cardiac surgery - replacing a patient's valve with a mechanical one.

Cristina spends all day preparing for it, and Teddy steps aside to let her learn on her own. The patient can't decide what valve she wants, so Cristina brings them both for her to try to get a feel for.

She finally decides on the porcine (pig) valve, and Cristina's rocking the house in surgery with Teddy looking on. Then things go incredibly bad. The patient is out of control, Cristina doesn't know what to do, and Teddy won't step in.

A worried Owen runs in to the OR to try to get Teddy to help, until Cristina throws him out. Then she figures out what to do, and she's fine. It worked!

After the surgery, Owen confronts Teddy, who swears she wouldn't have let the patient die, but correctly says that Cristina needs to be pushed.

Owen asks if Teddy didn't step in because of the situation between them. Owen goes to find Cristina, who's abolutely thrilled that she got to learn and fly on her own in the surgery.

And he crushes her a little by telling her Teddy's leaving Seattle. Cristina chases after Teddy and offered her anything she wanted to get her to stay - and agreed without hesitation to give up Owen to make it happen.

Finally, Mark's daughter, Sloan, is still living with him and Lexie, and he's embraced his role as a father, which is driving Lexie crazy.

More importantly, there's also a problem with her baby - a couple of pieces of amniotic tissue are threatening the tiny boy's limbs as they develop.

Get me Addison Montgomery, he says!

Understandably, younger Sloan knows she's ill-equipped to deal with raising a special-needs baby. But when Addison operates, she discovers an AVM (a tangle of blood vessels) that she's confident she can work around, but Mark shuts down the surgery, saying it's too dangerous.

His interference leads directly to two developments: First, he tells Sloan, without asking Lexie first, that he wants her to live with them and they'll help her raise the baby. Second, he takes Sloan to Los Angeles so Addison can operate.

Lexie tells Mark their relationship just ended, and goes back to Meredith's house, where she hooks up with Alex, who's there because he had to get out of the trailer.

Continued on the ensuing Private Practice episode "Another Second Chance" ...