On Quantico Season 3 Episode 10, Owen and Mike learn that a member of their family has been killed. For Owen, it's Lydia. For Mike, it's his sister.

Jocelyn notices strange wounds on Lydia's body, prompting her to do some research. She uncovers that similar wounds were inflicted on victims of IRA murders back in the '70s. There are 3 lines/wounds, which symbolize revenge.

Everyone immediately grabs their nearest family member and brings them to Shelby's mansion in Westchester for protection. Ryan brings his dad; Alex brings her mom, and Harry brings his sister.

No one gets along with their family member, but that's not all that surprising. 

Mike and an MI5 agent uncover a cell phone in Devlin's prison cell, and the team is able to track the one number that Devlin has called. Harry, Owen, and Jocelyn check it out, and they find a booby-trapped house with dozens of surveillance photos and information on Shelby's mansion.

At the house, an assassin poses as a neighbor to gain entrance into the house. He knocks out Shelby, and he tries to kill all of the family members. Thankfully, Ryan's dad manages to hold him off until Ryan can show up.

In the end, the team takes down the assassins. The MI5 agent informs Devlin that she plans on bringing him back to Ireland to face justice for his crimes.