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Episode: 112, "Crash Course in Polite Conversation"
What: After a harrowing night involving a storm and a possible death, relief turned to romance when Peyton and Lucas hooked up in their motel room. It was a a sweet moment -- which turned sour when they remembered that oh, yeah, Luke was still dating Peyton's BFF Brooke.
Why: Nothing like a little infidelity to remind you that the romantic lead of the show is a deeply flawed character. The magnetism between Luke and Peyton here serviced a running theme in their relationship -- the battle between their attraction to each other and their commitments to other people.