There's a reason why the Parks and Recreation gang is so sad in this scene: Ann and Chris have left for Michigan.
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Leslie throws Chris and Ann a party on their last night in Pawnee on Parks and Recreation. "Chris and Ann" is the 13th episode of the show's sixth season.
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Parks and Recreation Season 6 Episode 13 Quotes

I told him One Headlight by the Wallflowers isn’t dancing material and he told me, “Not with that attitude!”

Ann

Ann’s leaving town. Ann’s saying painful goodbyes. Greatest day of my life!

April