A Marine lance corporal, of Arabic lineage and the Muslim faith, dies two blocks from a mosque in Washington, DC; Gibbs and company investigate. Ducky uses an alternate method; Abby finds a deadly neurotoxin. A tip helps; justice prevails.
Someone has stolen new top-secret radar gear; the NCIS gets a tip about its location. During a stakeout of the site, McGee watches while a man dies; the radar disappears again. The gang and the Metro PD figure it out and catch the bad guy.
While seeking to serve a warrant, the Gibbs gang find cocaine, cash, and the dead body of the subject person; a dog bites McGee, who shoots the dog. Abby goes to bat for the dog, who helps to solve the case. The team nab the bad person.
Fornell and his Hoovers arrive in the squad bay; he tells that the Gibbs team is under investigation for the murder of the Frog. However, the Gibbs gang investigates the FBI, and they put the pieces together and explain them to the suits.
A Marine captain dies in a mortar explosion in Baghdad; the SecNav sends the body to Ducky, who finds that a bullet, not shrapnel, caused the death. Tony and Jardine investigate on site; they and the rest of the team together figure it out.
Ziva, undercover, goes on a date with a suspected serial killer (a Marine); Tony finds the fifth victim; the suspect threatens Ziva, who resists and shoots him with his piece. Gibbs and the team investigate, solve, and find a second killer.
Director Shepherd attends a funeral of a retired NCIS Agent, and notices someone suspicious. She gives Tony and Ziva time off and calls in Mike Franks for help. She knows someone is coming after her, and knows Gibbs will be next.
Jen died in a gunfight; Vance and Gibbs meet at the site; Gibbs leaves and meets Franks. Gibbs investigates unofficially; back at the office he suddenly leaves and goes to Jen's home, where he and Franks wrap things up. Vance makes changes.
A body is discovered in the water system on a Marine Base. A note stuffed inside the victim's mouth leads to a website showing how the first victim was killed. The team races to find the culprit because they know there will be more victims.
Gunmen rob the credit union at Quantico and kill a guard; Gibbs and company investigate; they question whether getting cash was the objective of the robbers. The team make several connections, then they round up the bad guys at a burial.
After an elaborate setup Tony and Ziva break into a classified Navy facility; however, that test is itself a ruse by which to identify a mole within the NCIS. Gibbs and Lee meet in the chat room, and Gibbs says that now they work together.
Tony and McGee unsuccessfully try to follow Lee's pickup man; a satellite leads the gang to a house with a secret room and a dead body; a second stakeout produces a different result, which takes the team to a solution, a catch, and more.
A boatswain's mate dies after a single-car crash; Gibbs and company find evidence of gunfire plus beatings of the victim, who was a member of a fight club and, they learn, the operator of an illicit business. The gang figure out all of it.
A Metro detective investigates a burglary and double homicide at Christmas in Washington; the suspect is a former Navy petty officer who supposedly died 17 years ago; the Gibbs gang take an interest, find the pieces, and put them together.