The owner of a diner shows up at his workplace, only to find five dead bodies on the floor.
As the team begins to investigate, two of Gibbs' ex-wives - Diane Sterling and Rebecca Chase - show up at the crime scene, demanding to speak with him.
It turns out that all victims had a few things in common: they all failed to report for duty the week before, plus all were late in paying their taxes.
The crime scene appears to show that the four male victims had surrounded the female, who had gunned them all down. It appears she also had been shot and had died of blood loss.
Bishop discovers that all victims had scheduled appointments to see a tax consulting firm the weekend that they disappeared. The name of the firm is "We Got Your Back Taxes"
When Gibbs heads home, Rebecca tracks him down and barges into his home. She then tells him that she's there after "bottoming out" from alcohol, and that she's now in AA and is following one of the steps of the program, which involves making amends. She apologizes for the hurt she caused Gibbs by sleeping with another man while they were married.
Gibbs forgives her.
When he complains about a massive headache he has (from not drinking coffee), she offers to massage his head. They eventually fall asleep on his couch.
The next morning, McGee and Gibbs' first ex-wife Diane discover them asleep and cuddled together on the couch.
Diane informs him that the IRS had been hacked, with the intent of discovering records of Navy sailors who were late paying their taxes.
Before leaving Gibbs' house, Rebecca gives Gibbs a wedding invitation and asks him to attend.
McGee and Gibbs break into the firm of "We Got Your Back Taxes" and discover a mostly empty room. They also find a bullet on the floor and some blood - which Abby then informs them comes from the five victims in the diner.
Abby hypothesizes that the diner scene had been staged, and that all victims had actually been shot to death in the fake tax office and then transported to the diner.
When another Navy officer is found dead in the street, victim of a stab wound from a scalpel, Abby realizes that both murder scenes are identical to key death scenes in Gibbs' life. The diner scene was staged exactly like the scene of Jenny Shepard's murder scene, and the dead man in the street was staged exactly like Mike Franks' death.
Clearly, someone is recreating murders from Gibbs' past.
The team believes that the re-appearance of Rebecca in Gibbs' life might have something to do with the crimes, so they investigate her and discover that $300,000 had been deposited into her account in the Cayman Islands. They also trace back the I.P. address of the computer that hacked the IRS and find it belongs to Rebecca's computer.
When Gibbs brings Rebecca in for interrogation, she includes her lawyer who happens to be Gene Isaacson. Gene is not only her lawyer, he's her fiancé as well. He's also the man with in bed with Rebecca when she cheated on Gibbs.
Rebecca explains that the $300,000 was an inheritance. She theorizes that she's being set up.
Abby investigates Rebecca's phone and agrees that she's being set up when she finds a bug in it.
After uploading the schematics of the bug to the FBI's ViCAP database, she finds a connection to a man who works with Sergei Mishnev.
The team traces Mishnev to an RV in a trailer park. After busting in, they find the walls plastered with pictures of Rebecca.
Gibbs receives a phone call that seems to be coming from Diane, who informs him that she needs to see him immediately, and that she's texting him the address.
After telling Tony to bring Rebecca and her fiancé into protective custody, Gibbs meets Diane in a parking lot.
She doesn't know why she's there, and tells Gibbs she didn't call him.
Suspecting a trap, Gibbs tries to tackle her but a shot rings out, and hits her in the forehead, killing her.
Gibbs spots Mishnev getting into a black SUV. He fires at it, but doesn't hit anyone.
McGee tells Gibbs that the RV had been rigged, and that a hidden computer had been set up to call Gibbs' phone, and that instructions were given to him to meet Diane - but that the voice wasn't her's - though it had been rigged to sound like her.
On the computer was also a video recorded message from Mishnev to Gibbs, gloating about his plans and about he had set the whole thing up to murder Diane.
Abby discovers that Mishnev had been able to obtain Gibbs' personal records, as well as the records of everyone he's worked with, and that this had been done as part of the payload from the virus that infected the NCIS building back in NCIS Season 12 Episode 1.
She figures that if she re-engineers the virus to remove its system lethal capacity, and then reactivates it - it would act as a homing device back to Mishnev.
Abby does so and obtains an address for Mishnev, which she then tells Gibbs as he's driving. He turns his car around and approaches Mishnev's apartment by himself.
Upon breaking in, he shoots most of the inhabitants except for Mishnev, who quickly raises his hands in surrender.
Gibbs begins to beat him up, but is then knocked unconscious by another of Mishnev's cohorts.
Gibbs wakes up to find McGee, Tony and Bishop standing over him.
A call comes in on Gibbs phone. The call displays says "Fornell"