Unresolved issues between Ben and Serena come to light when his mother visits. Meanwhile, Russell puts his relationship with Raina at risk and reveals a tragic secret about Bart to destroy Lily and the Bass family legacy. Nate and Raina explore their feelings for each other, and 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. Plus, Dan and Blair's relationship reaches a turning point that changes everything.