Dan invites her to Blair's birthday so she can reveal a tape of Blair singing "Stand By Your Man" at a Robyn afterparty in Stockholm. It's revenge for Blair and Chuck banishing his sister Jenny from returning to Manhattan. That makes sense, right? Robyn then performs the acoustic version of "Hang With Me" while Dan reveals his treachery, and as Serena and her professor (you might know him as Dr. Rapist on Mad Men) talk about how much they want each other's bods upstairs in her bedroom. Later, Robyn's "Dancing On My Own" plays as Serena and Dr. Rapist make out, while Blair and Chuck have hate sex on her mother's baby grand.
Fortunately, Robyn's appearance also involved some singing, which you can see here:
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