On the case of the week:
Police officers discover a boy named Henry Marshall sitting in a closet, holding a gun. He tells them he saw an intruder enter his home with a knife, so he got his father's gun out of the safe and shot him. The intruder escaped.
Abby finds a DNA match for the blood of the intruder found at the house. His name is Mickey Doyle.
She also discovers that the safe that Henry said he ran to get the gun had been opened two days earlier. He lied.
Henry tells McGee that two nights ago he witnessed Mickey Doyle stab another man to death, and that Doyle knew he was a witness. Henry got the gun out of the safe that night and has been holding onto it ever since.
Henry's mother Anne can't practice law because her new state doesn't recognize her law degree credentials from the state where she passed the bar. Bishop wants to reach out to an organization called Joining Forces which supports military families. She wants them to advocate for Anne.
Henry sees a story about a man named Wayne Woods who was found floating in a river. He tells McGee he's the man he saw Doyle stab to death.
Gibbs, Bishop and McGee track Doyle to his RV. They storm the camper and find him dead.
McGee discovers that Henry was tracked to his house via traffic cameras, which was done by someone in the police department.
Gibbs tells Detective Swanson that one of his officers was on the take with the drug gangs in the county, and was responsible for killing Doyle.
Gibbs and Bishop arrest Officer Kristen Fields.
On the end of season arc:
Gibbs has sent Tony to England to work with MI6 to hunt for information on Jacob Scott. He has since gone to follow a lead in Russia.
Vance and Fornell head to London.
The list of targets from the Juniper Strike operation only contains the names of NCIS agents. The names of MI6 agents in the operation have been redacted by the chief of MI6, Jessica Terdei.
Terdei has not been seen since her retirement in 2008. Tony located an application form she completed in 1984. It lists a man named Hugo Radcliffe as her emergency contact.
Hugo Radcliffe takes Vance and Fornell to the cottage where Terdei has been staying.
Terdei tells Vance and Fornell that an unknown sniper was responsible for shooting Nika Razin, Jacob Scott's wife. She says she conducted an off-the-books investigation into MI6 to determine who leaked the details of the operation. She determined the leak must have come from a mole within NCIS.
She tells Vance that she voiced her suspicion to Tom Morrow, but that Morrow buried it.
Shooters show up at the cottage, and begin firing. They take off.
Fornell and Terdei arrive at Gibb's house. Fornell opens the refrigerator, and notices a blood spot on the wall next to it. He turns and notices that a door has been broken open.
He brings beer back to Terdei, and subtly warns her there's an intruder. They take their guns out and turn back to the kitchen but the intruder has the drop on them and he shoots them both.
The shooter walks over to Terdei and shoots her again, killing her.
Fornell's phone begins to ring. It's his daughter calling. He reaches for the phone but then passes out.