Peter Cordero a futures trader at JP Harding is shot and killed in his home. Lanie believes he was wearing a wire which was removed.
Capt. Gates sister Elizabeth is the US Attorney who had Cordero as a confidential informant but she and her sister do not get along. Several years back, Elizabeth asks Gates not to arrest a dirty cop who was a key witness in an undercover operation. Gates did her job and arrested the officer and her sister never forgave her.
Turns out that Elizabeth's partner had set up Cordero on a drug charge in order to turn him as an informant because she thought Elizabeth wanted her to do anything for the greater good. Then she had to kill Cordero when she realized he was going to run and the truth about what she'd done could come out.
Rick and Kate stress out over the huge guest lists they've assembled. In the end they realize having the important people in their life at their wedding is more important.. Gates estranged sister becomes a part of a murder investigation when her confidential informant is murdered. We'll recap the episode in full after it airs.