5.16 An Honest Mistake
The pregnant woman needs emergency surgery, but her husband tells Derek and Addison that if they're forced to choose, they should save his wife and let the baby die. During the surgery, things go wrong for the mom, but Derek still feels guilty for his mistake so he does everything he can, including cutting out large chunks of her brain. Addison thinks the woman is dead and wants to remove the baby, so she threatens to perform a C-section while Derek prepares to cut out the mom's frontal lobe.
The Chief arrives to settle it and he sides with Addison. She cuts out the premie while Derek looks on with the sudden realization that he's not a god. When they tell the husband, Derek can't bring himself to say anything other than "I'm sorry." The husband gets furious and calls Derek a murderer.
Lexie is tired of Mark Sloan hiding in the attic like Anne Frank, and she wants him to tell Derek about their relationship so she can feel free. When he finally does tell Derek, he punches him. Then Derek punches him again. Then Sloan retaliates and it turns into a huge beatdown with punches and blood. Owen finally rushes in to stop this senseless nonsense.
At the end of the day, Derek downs a glass of tequila, then pours one more for him and one for Meredith, and as the camera pans out, he drinks and pours again.