G-Girl, Season 4 - Episode 19: Petty in Pink

Serena (Blake Lively) enlists Charlie (guest star Kaylee DeFer) to go undercover Gossip Girl-style to determine why Dan (Penn Badgley) and Blair (Leighton Meester) are spending so much time together, and what role an unexpected visitor is playing in their lives.

With Nate’s (Chace Crawford) help, Raina (guest star Tika Sumpter) decides to go on a search for her long-lost mother. Chuck’s (Ed Westwick) hopes about his legacy are dashed with the reveal of devastating information from the past.

(Source: TV Fanatic)