Community service workers find Marine Lance Corporal David Austin dead from a motorcycle accident.
Ducky later discovers that Austin was shot in the chest before his accident.
Abby finds that the bullet was fired from an FN57 pistol purchased by the ATF. Zoe tells Gibbs and Vance that the gun was likely part of a sting operation called "Fast and Furious", in which a shipment of guns were sold by licensed arms dealers to straw buyers -- people who purchased guns on behalf of other buyers who weren't legally allowed to buy them. The ATF had hoped to track the guns to Mexican drug cartel leaders.
A total of 2000 firearms were purchased by straw buyers. Of those, only 700 were recovered, leaving 1300 unaccounted for.
The team discovers that before he was shot and killed, Austin had reacted to a call of distress by a girl in a car, and had taken off after her when the car left. It was a result of that action that the man in the car had killed him.
McGee determines that the car was registered to a woman named Jennifer Vickers - but she's not the woman in the car.
ATF Special Agent Phillip Caffey tells Vance and Keates via MTAC that he had issues with the way operation "Fast and Furious" was being run.
Bishop and Gibbs confront the owner of the car, who tells them that her daughter Emma was using the car. She tells Gibbs that her daughter had been in and out of rehab and had gotten involved with some bad people, and that she had moved out of their apartment.
Zoe tells Tony and McGee that Caffey couldn't find anything on the gun. Tony believes he's stonewalling - and McGee agrees.
The car is found with the dead body of Emma Vickers in it.
Abby gets a DNA match from Emma's body that matches a man named Jamie Rivers, who has a record for assault and battery. He was also arrested in 2008 for the illegal sale of firearms.
In a picture of Rivers' arrest, Vance notices that one of the arresting agents is ATF Agent Phillip Caffey, Zoe's friend.
Caffey tells Gibbs and Vance that he arrested Rivers just prior to the "Fast and Furious" operation. He cut him a deal to work with the ATF as a straw buyer. He says Rivers told him that the gun (later used in Austin's murder) had gone to the Mexican cartel. He says that after the sting failed, ATF lost all contact with Rivers.
Zoe gets hold of a local informant to see if he knows whether Rivers is operating in the area.
Zoe tells Vance and Gibbs that Rivers has been selling guns in the area, and is now using drugs. Gibbs suggests that Zoe set up a buyer for Rivers.
McGee poses as the gun buyer.
Rivers shows up, but then notices a couple of cops on horses. He runs. When Tony challenges him, he turns and fires. Gibbs shoots him dead.
In a gesture of gratitude for Austin's attempt to save her daughter, Jennifer Vickers tells Ducky she wants to donate Emma's trust fund to the Marines, in Austin's name. Ducky suggests that the Marine Corps Scholarship Foundation would be a good place to put the funds.
Meanwhile, Tony Senior pays a visit to Tony because he wants to meet Zoe.
Tony and Zoe get to his apartment, to discover that Senior caused a fire in his oven. He also finds out that Senior has re-arranged his furniture.
After having a shouting match, Senior leaves and shows up at Gibbs' place.
Senior tells Gibbs that he went to Europe and tried to "get back in the game", but all of his contacts were gone. He says he went to New York but all of his friends were gone. He tells Gibbs that the only person who matters to him is his son.
Gibbs tells Senior that he regrets not trying harder to get his father to move in with him before he died.
Gibbs brings Tony back to his home and tells both of the DiNozzos to figure things out and get out of his house.
Tony apologizes to his father for what he said, and tells him that he understands why he came to see him - that he wanted to move closer to him.
Senior has a discussion with Zoe about Tony's commitments issues which scares the crap out of her.
Senior tells Tony that he found an apartment right above Tony.
Later, Senior tells Tony he's reconsidered, and won't be moving in above him, to Tony's relief.