A mystery man waits for Agent Fornell across from the drive through where he's getting dinner. As Fornell digs in in his car, the man crosses the street toward him and fires twice through the windshield. He looks to see if he hit Fornell and is knocked down by Fornell's hail of return fire.
One the scene later, Fornell tells Gibbs he's fine. He had a bullet proof vest on after work. The shooter was Navy seaman Tyler Brown. Fornell doesn't recognize him. Duckie checks his pupils and thinks he might have been on a stimulant.
They find the matchbook with Fornell's license plate in the shooter's car.
Fornell and Gibbs visit a Navy bar, where Brown worked as a bouncer. A drunk bar patron says Brown liked beating people up and was dealing speed.
Gibbs and Fornell are both taken aback when their mutual ex-wife Diane (Melinda McGraw) walks out of the bathroom.
She's an IRS auditor and says she's working, investigating the bar. Gibbs stops Fornell from telling her about the shooting. When the bartender comes back, he calls Melinda "Mrs. Fornell" and says the owner isn't interested in selling the bar. Gibbs is annoyed.
They take her back to NCIS and put her in interrogation for obstructing their investigation. She had two IDs on her, one in the name of Gibbs and one in the name of Fornell.
The victim was reprimanded for fighting and was in the process of dishonorable discharge.
Gibbs makes Tony question Diane, who is not happy about it. She says her husband Victor is at a destination wedding with her daughter.
Vance interrupts, ending the interrogation.
Diane calls her boss from Vance's office and gets clearance to talk. She's not an auditor, she's in enforcement one of the IRS's agents.
"Agents? And the Lord wept," Fornell says, aghast.
She was undercover in the bar. Someone is stealing identities from the bar. She used Fornell's name because fake IDs are expensive. Fornell is angry her investigation using his name almost got him killed.
Vance announces the three of them will be working the case together.
Gibbs and Fornell go to check up on fake tax return checks.
Diane and Ziva go to check cashing outlets. She mentions she's taking a break from her husband. She wonders if made a mistake getting married again, missing her chance to go out and act crazy.
They show the check cashing clerk (Danny Nucci) the fraudulent checks. He recognizes them as belonging to Oliver Lambert. He gives them surveillance footage.
Back at the office, they figure Lambert is the ID theft ringleader. They realize Diane could be in danger and argue over who has to watch her.
She ends up at McGee's, complaining that her husband wants to leave her.
She worries there's something wrong with her. McGee assures her she's attractive, "at least on the outside."
She asks McGee for a hug. He balks, so she insists. When she says his parents failed, he relents.
In the morning, Gibbs and Fornell find McGee, asleep, spooning Diane on the couch. "Holy Fourth of July weenie roast! What the hell am I looking at?" Fornell demands.
McGee wakes up protesting it's not what it looks like. Gibbs reports Abby has a lead on the accountant, an overdue payment on a rental house.
The men meet Tony at the house.
They bust in and find a room converted to a walk-in freezer with chests full of seafood. "Right, of course. The accountant was renting a home for his frozen fish collection," Tony concludes.
The drunk guy from the bar is in another chest.
Back at NCIS in the morgue, McGee protests to Duckie, Tony and Palmer that nothing happened with Diane.
Duckie reports the freezer victim was Viggo Kiln, working with Oliver the accountant to chat bar patrons up for details to steal their identities.
Upstairs, Ziva and Diane work on a theory about the fish. Diane finds a shipping manifest for a shipping company in China on the border with North Korea. Others were going to countries on the borders of Syria and Iran.
Diane goes to talk to a customs agent she knows. She sends Fornell and Gibbs to look into the shipments. They're miffed that she's acting like she's in charge.
In the lab, Abby opens up a tuna to show Ziva the fish had a cell phone in it. All the fish had them, from the same manufacturing batch.
Someone used one to call Oliver and she tracked the caller's location to a park.
They stake out the park. Fornell and Gibbs gossip about Victor maybe leaving Diane. Fornell tells Gibbs that Diane asked to be transferred to Gibbs' jurisdiction, probably hoping to run into Gibbs.
Oliver Lambert sits down on a park bench and Gibbs and Fornell approach him. He's zonked out and won't take his hands out of his pockets. They all draw on him as he says he was trying to set things right.
"I gave them this week's payment, but they're worried, I think they might try and kill me," he says.
He takes his hands out of his pockets and shows he's bleeding badly from his stomach. He keels over.
Back at the office, Duckie confirms Lambert died of multiple stab wounds.
He had thumb drive in his wallet showing he was embezzling. He had a statement prepared to give to the Treasury, but with his boss' name left out.
Abby, with Palmer's help, was able to track down a shipment of fish that had DNA similar to the fish they found. Using this information, they were able to track other shipments of fish. They get a call about one shipment that had DVDs in the fish and another with lavendar soaps. Apparently these American items are hot commodities. Abby reports she traced the fish to a distributor run by Avis Boyle.
Diane joins them, saying he was a client of Oliver Lambert's. They think he's the smuggler, but don't have enough to arrest him.
Diane points out that with Oliver dead, Boyle will need a new accountant to launder his money. She wants to approach Boyle. Gibbs insists on going with her.
Boyle is at a wedding.
Fornell grills McGee on his couch cuddle time as they run surveillance from the van.
In the wedding, Diane sees Boyle and approaches him. She lays out her pitch and Avis goes to get his boss. On surveillance, they see the boss come in. It's the guy from the check cashing place (Nucci). Problem is, he knows that Diane is an IRS agent.
He pulls a gun on Diane and starts to lead her out when Gibbs gets everyone's attention by proposing a toast.
Then he gets Diane away from the check casher by getting down on one knee and proposing to her in front of everyone. The team swoops in and arrests the man.
Back at the office , Diane explains to Vance that Boyle was one of four distributors in the smuggling operation. The check casher was Gordon Fremont, running the whole thing.
Down in his basement, Gibbs gets a visit from Diane. She's touched he remembered how he proposed and repeated it.
Gibbs tells her to give Victor a break "for being him and not somebody else. You've got to let it go, Diane. I couldn't and I drove everybody away. The last thing you want to be is me," he says.
She assures him he's not so bad. She asks if he'll spend the rest of his days alone.
"I'm not alone," he says, putting his NCIS cap on.