: Chuck and Jenny team up to uncover the truth about Lily’s medication. They eventually enlist Blair, who uses a black-tie event to trap Dr. Holland—the neighbor William used to write fake prescriptions.
The episode focuses on the group's efforts to expose William van der Woodsen's true motives. While Serena remains fiercely loyal to her father, Rufus feels increasingly alienated from the family.
"Gossip Girl" Ex-Husbands and Wives (TV Episode 2010) - IMDb
: As the truth comes to light, Jenny tips off William that the group is on to him. William manages to flee New York just as the police close in, but not before telling Serena he truly loves her and Lily. Romantic Arcs and Ultimatums
: It is discovered that Lily was not actually sick; William had been providing her with unnecessary medication to keep her dependent on him and to win her back from Rufus.