In this sneak peak from "The Becoming," we see the spark between Meredith and Derek return at last.
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A promo for the 14th episode of Season Four of Grey's Anatomy, "The Becoming."
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Grey's Anatomy Season 4 Episode 14 Quotes

I slept with her... and the whole time I was thinking of Meredith... who's the whore?

Derek

(to Meredith) If you want me to stay, I'll stay.

Derek