‘Blue Bloods' Season 6 Episode 21 spoilers: A terrified witness on the run makes Erin go ‘The Extra Mile’

Fans are in for another action-packed episode of “Blue Bloods” on Friday. The highly popular series returns on April 29 with its second to the last run for season 6, titled “The Extra Mile.” In the upcoming installment, Erin (Bridget Moynahan) leads a man-hunt to track down a key eyewitness in a murder trial, who flees out of fear of retaliation. Spoilers hint that Erin, along with Anthony (Steve Schirripa), will try to convince the witness to testify in court, which could prove crucial to the case.

“Blue Bloods” followers wonder why episode 21 seems to be a bit toned down compared to the previous run, but they hope surprising twists are ahead. The upcoming installment will also feature other “Blue Bloods” cast members including Frank (Tom Selleck), Jamie (Will Estes), Danny (Donnie Wahlberg), Henry (Len Cariou), Linda (Amy Carlson), Nicky (Sami Gayle), Det. Maria Baez (Marisa Ramirez) and Officer Eddie Janko (Vanessa Ray).

Spoiler alert! This update contains more "Blue Bloods" spoilers. Read on at your own risk if you have not watched the latest episode.

According to TV Guide, Garrett pushes Frank to attend the Police Union party, while Danny and Baez delve into a baffling robbery case that happened in a convenience store. Elsewhere, Erin and Anthony trail a witness, who because of fear runs away before giving a testimony in a murder trial.

CBS has posted a sneak peek of the “Blue Bloods” April 29 episode on its official site. The teaser poses a question that goes, “Will a prosecutor risk a life to win a case?” Erin and Anthony are seen arguing about putting the witness to the stand. “My job is to get a dangerous killer off the street,” Erin says.

The next scenes show how the witness running from one place to the next, in an attempt to escape the police. On the final part of the trailer, Erin begs the witness to testify. The man tearfully replies, “I’m begging you.” Erin, however, tells the police to take him away.

“The Extra Mile” is directed by John Behring and written by Brian Burns. KSite TV also named other actors and actresses who will be seen in "Blue Bloods" Season 6 episode 21. The upcoming run will feature recurring guest stars Tony Terraciano (Jack Reagan), Andrew Terraciano (Sean Reagan), Abigail Hawk (Abigail Baker), Gregory Jbara (DCPI Garrett Moore) and Robert Clohessy (Lt. Gormley). Also appearing in the upcoming episode are Patch Darragh (Joe Kaplan), Stephen Rowe (PBA President Johnny Lyons), Brandon Gill (Willis Stratton), Philip Reid (Dontrell Tariq), Julee Cerda (Defense Attorney), Pamela Dunlap (Judge Dolan), Hassan Johnson (Morgan Rutherford), Petronia Paley (Josephine Stratton), Nathaniel Stampley (ESU), Erika Woods (Nurse), Matt Dellapina (Doctor) and Emily Coffin (Mrs. Kaplan).

Find out what happens next on "Blue Bloods” Season 6, which airs in the US every Friday at 10 pm on CBS. In Australia, “Blue Bloods” airs on Thursdays, 10:30 pm on TEN.