<P> Unresolved issues between Ben and Serena come to light when his mother visits . Meanwhile, Russell puts his relationship with Raina at risk by using personal information to blackmail Lily in order to destroy Chuck's family legacy . Russell tells Chuck a devastating secret about his father . Blair and Dan's relationship continues to build . Nate and Raina explore their feelings for each other . Lily prepares to face legal consequences for her part in Ben's false imprisonment . William van der Woodsen makes his return to the Upper East Side . </P> <Tr> <Th> 83 </Th> <Td> 18 </Td> <Td> "The Kids Stay in the Picture" </Td> <Td> J. Miller Tobin </Td> <Td> Jessica Queller </Td> <Td> April 18, 2011 (2011 - 04 - 18) </Td> <Td> 1.43 </Td> </Tr> <Tr> <Td_colspan="7"> <P> William, along with Lily's mother and sister, CeCe and Carol (Sheila Kelley), come to New York to support Lily as she awaits the consequences of confessing to her crime . Carol's daughter, Charlie (Kaylee DeFer), who has been raised away from the glamour and wealth of the Upper East Side, follows her mother to the city to meet the rest of her family . Meanwhile, Blair and Dan deal with the aftermath of their kiss as Chuck realizes that he wants to win Blair back . </P> Title reference: The 2002 film The Kid Stays in the Picture . </Td> </Tr> <P> William, along with Lily's mother and sister, CeCe and Carol (Sheila Kelley), come to New York to support Lily as she awaits the consequences of confessing to her crime . Carol's daughter, Charlie (Kaylee DeFer), who has been raised away from the glamour and wealth of the Upper East Side, follows her mother to the city to meet the rest of her family . Meanwhile, Blair and Dan deal with the aftermath of their kiss as Chuck realizes that he wants to win Blair back . </P>

What episode do dan and serena get back together in season 4