'Blue Bloods' season 6 spoilers: Erin helps in finding a missing witness to a homicide in episode 9

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Erin Reagan (Bridget Moynahan) will once again use the skills that she learned from her family in the next episode of "Blue Bloods" season 6.

In the episode titled "Hold Outs," the only daughter of NYPD Commissioner Frank Reagan (Tom Selleck) will be forced to assume the responsibility of her colleague Kelly Blake (Diane Neal) when a homicide case results in a hung jury. But to provide stronger evidence for the jury, she needs to find a missing witness who will provide vital information for the case. However, Erin will not do it alone because she will be assisted by DA investigator Anthony Abatemarco (Steve Schirripa).

The episode will also feature another undercover operation as they disguise as an elderly couple to investigate a gang that spreads fear in a neighborhood.

In the previous episode, Jamie (Will Estes) encountered problems with his immediate superior when he chose to disobey the orders of Sgt. William Mulvey (AJ Buckley), who wanted to attack a hostage taker without calling for backup. Jamie decided to follow the proper protocol, but he almost got reprimanded for doing the right thing. But it was later revealed in the episode that Sgt. Mulvey is actually bypassing to have a chance to die while on duty to secure his family's future because he is already dying due to pancreatic cancer.

Ultimately, Sgt. Mulvey was removed by Lt. Gormley (Robert Clohessy) because the department cannot afford to keep an officer who does not care if his life or the other people around him is put in danger.

The upcoming episode will showcase Jamie and Eddie's skills in going undercover, which happens to be their expertise.

The official synopsis for the upcoming "Blue Bloods" season 6 episode reads:

"When a homicide case ends in a hung jury, Erin is forced to take over the case from her colleague, ADA Kelly Blake, and with the help of DA investigator Anthony Abetemarco, find a missing witness. Also, Jamie and Eddie go undercover as an elderly couple to catch a gang that is terrorizing a neighborhood."

The episode airs on Friday, Nov. 20, on CBS.