Carrie's birthday and her angst about turning 35 is what drives the episode, but that only serves as backdrop for the most important moment of the series, in my opinion. The end of the episode culminates in a revelatory scene at the coffee shop, where Charlotte essentially articulates the show's theme in the later (and better) seasons- "maybe we could be each other's soulmates and then we could let men be just these great, nice guys to have fun with."
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