Dochází k výbuchu. Auto, které převáží mrtvé tělo Williama Danfortha Juniora, který zemřel při automobilové nehodě, vybuchne. Detonace byla spuštěna z dálky a zřejmě až po té, kdy oba řidiči z vozu utekli. Oheò zničil Danforthův chrup i DNA, přesto se však Duckymu podařilo z části tkáně DNA identifikovat a prokázat, že DNA mrtvého se neshoduje s DNA Danfortha v armádních záznamech.

Pátrání se přesouvá k Danforthovu příteli, Kennethu Merillovi. Abby se podaří zjistit, že se Danforth s největší pravděpodobností podrobil transplantaci a to by vysvětlovalo i fakt, že DNA není totožná. Danforth senior přiznává, že synovi byla transplantována ledvina a dárcem byl jeho nejlepší přítel Kenneth Merill. Rodina se rozhodla držet tuto informaci v tajnosti, protože ani dárce ani příjemce by už nemohli nadále zůstat v armádě. Test DNA při pitvě by vážně ohrozil Merillovu další kariéru. Proto se otec rozhodl zaplatit za prohození mrtvých těl a přiznal se, že on sám inicioval explozi.

Two paramedics are transporting a dead body from the morgue of a D.C. area hospital to the Bethesda National Naval Medical Center (NNMC). The remains they are carrying are those of a Marine who was killed in a car crash. Shortly after their ambulance departs, the driver smells something unusual. His mate confirms that it must be gas. As soon as they stop, the vehicle suddenly ignites. The two men have barely sufficient time to run away before the explosion of the truck.

At NCIS HQ, Tony who is using McGee’s computer is chatting with the very cute NCIS Special Agent Tina Larsen who is there TDY for a couple of days. Tina believes she is on-line with Tim and talking about his coworker Tony who has been trying to flirt with her earlier. Ziva is amused by what’s going on but warns Tony that he will be in serious trouble if Gibbs catches him playing on McGee’s PC. When Gibbs walks in unexpectedly almost at the same time as McGee, Tony and Ziva start improvising a phony and funny celebration of Tim’s birthday. As they are leaving the room, following Gibbs to go investigate the explosion of a medical transport, Tina is just inviting the one who she believes is McGee for a drink…

On the terrain, Tony and Tim persist in their discussion about McGee’s unanticipated date while Ziva is trying to make them focus on their work. Tony who must be feeling somewhat guilty becomes sarcastic about Ziva’s expertise. Ziva brings her two dissipated colleagues back to reality when she replies that, being Israeli, “this is not her first exploding ambulance…” At the same time, the two paramedics explain to Gibbs that they haven’t left the van unattended for more than 4 or 5 minutes. One of them thinks that somebody must be trying to kill them. When Gibbs asks “Who was trying to kill you?” the co-driver replies: “Who knows, there are a lot of weirdos out there!” And Gibbs replies: “Tell me about it”.

Ducky tells Gibbs that the remains that burned with the ambulance are those of Lance Corporal William Danforth Junior who survived IED (Improvised Explosive Device) in Iraq and RPG (Rocket Propelled Grenade) in Afghanistan… but unfortunately passed away in a car accident trying to avoid a deer at 60 miles an hour. Ducky says this could have been an accident, lucky for the two escorts to get out… Gibbs: “Lucky in a way I don’t like…”

The next day, in the office, Gibbs surprises Tony who is cheekily browsing photos inside McGee’s private IPod that the poor Tim imprudently left on his “public” desk. They both receive Gibbs traditional “slap” on the head: Tony because of his bad behaviour and Ziva for alerting him of Gibbs’ arrival. More seriously, Ziva doesn’t think that the drivers were the target.

A few minutes later, Abby is explaining what she has discovered with regard to the ambulance fire. This was most likely not an accident. Moreover, McGee presents evidence of what could have been the trigger of the explosion of the Oxygen canisters. It also seems that the person who triggered the explosion waited long enough for the two drivers to be able to escape. Gibbs also questions if the DNA of the Marine was confirmed? Abbs answers him that she has been very busy so far but that Ducky is taking care of that.

In autopsy, Ducky is puzzled by a mystery. He explains Gibbs that the death certificate of the Marine indicates that he died from “blunt force thoracic trauma” which is typical in motor vehicle accidents but that he has only been able to partially verify that. Very professionally, Ducky has attempted to identify himself what is left of the body but the young man’s teeth are in such a bad state that it would take a very long time to make identification from dental records. Also, his DNA has been extremely degraded by the fire but, nevertheless, Ducky has found a piece of tissue viable enough to run a test against Danforth’s DNA at the Armed Forces repository. The DNA is from someone else: that is the mystery!

Gibbs goes back in the office and informs Ziva and Tony that the fire may have been caused purposely to hide the real identity of the victim. For this reason, Gibbs sends Ziva and McGee to the hospital to interview the doctor who pronounced the Marine dead and find out who identified him. He is then told by Tony that the Director needs to see him in her office ASAP. Gibbs enters Director Shepard's office abruptly and unannounced in a form of retaliation for the term “ASAP” but realizes he somehow goofed because she is not alone.

The Director introduces him to Mr. William “Bill” Danforth, the father of the dead Marine. Danforth is himself a former member of the Corps and a friend of the SECNAV. He admits that he is using his personal relation to be kept posted on the progress of the investigation. He wants to know how the body of his son was incinerated. Gibbs informs him and Director Shepard of “the mystery.” The body found in the ambulance’s fire is not the one of his son. Since Bill was a fellow Marine, Gibbs promises to keep him in the loop. Before letting him go, Gibbs asks Bill if he identified his son’s body. The latter denies. When the father asks in turn if it is possible that his son could still be alive, Gibbs answers that the only thing he knows for sure is that the hospital transported the wrong body.

At the hospital, McGee and Ziva are speaking to the doctor who pronounced Danforth dead. He says that when the victim arrived he was in a very bad state and hard to recognize. When Ziva asks who identified his body, the physician says that they took his driver’s license from his wallet but also that a friend confirmed his identity. When asked who this friend is, he points them to a young man reading a book aloud for a young lady lying in bed badly injured; she is Rebecca, Danforth’s girlfriend. Corporal Kenneth Merrill confirms he helped identify the body. His buddy was in his own car with Rebecca. Before, the three of them had been in a bar “celebrating” the end of Billy’s second tour in Iraq. Merrill didn’t drink but Billy was drunk and Rebecca was asked to drive him back home. Merrill also explains, he and Danforth had been friends since they were children. He had discovered the accident when went back home 30 minutes after the couple had left.

Gibbs went to interview Danforth’s CO while DiNozzo was sent to discuss with Billy’s platoon. In the camp, Gibbs and Tony are crosschecking their results. It appears that Danforth was a top Marine but the word is that he only signed in because his father was a Marine before him. William was inseparable from a certain Corporal Kenneth Merrill, an IED specialist, who is known in his platoon as the “Forrest Gump” of explosives. Tony asks Gibbs if he is suspects Merrill of helping Danforth to fake his own death to get out of the Corps. Jethro answers "No, I’m wondering who they got to play the part of the dead body!"

Taking advantage of a momentary absence of Gibbs, Director Shepard, who wants to keep the SEC-NAV happy, comes down from her office to discuss the investigation with Gibbs’ team, fishing for some information. She learns that they have two opposing theories. Tim and Ziva think that Corporal Merrill may have killed his friend because they had a fight in the bar which Kenneth failed to mention and that since Merrill passed by the accident scene he had the possibility to finish Danforth off. They believe that there could have been a third person in the car. Merrill went to hide Danforth’s body. When he came back, the cops had already arrived. He had to go to the hospital to confirm Danforth’s ID but would need to burn the ambulance to avoid discovering the body's real identity during autopsy. Tony's theory includes a motive. He believes that while driving drunk instead of Rebecca, Danforth did not hit a deer but a human being. To avoid admitting the accidental killing, he swaps “his” body with John Doe's, then escapes and hides himself. He has his friend Merrill confirm the ID and later blows up the ambulance. Gibbs returns and is annoyed that the Director has spoken directly to his team without him.

Gibbs and Ziva go back to the unit to confront Corporal Merrill. They try to make him elaborate about the fight in the bar. Merrill regrets that he was unable to take his friend’s car keys away. He is also sorry that he did not drive Danforth back himself. He says that Billy died because of him. He seems amazed that the burned body is not Corporal Danforth. Tony and Tim go back to the accident scene where they find a prescription bottle belonging to Corporal Merrill. There is blood on it.

The next morning Tony explains to Ziva that this discovery proves that his bodies swap theory is the good one. He is interrupted by Gibbs who says that Abby found Danforth’s blood all over the windshield and the hood. Ziva insists that Merrill was the only person with sufficient experience and means to blow up the ambulance. Gibbs says she is right.

Abby and McGee have discovered that the fire in the ambulance was triggered by a disposable cell phone. There is no way to read the call log of what remains of the phone, but McGee pulled the list of calls placed in the hospital cell zone at the moment of the explosion. One came from the cell phone of Corporal Merrill.

Gibbs meets William Danforth Sr. at the hospital and explains that the fire was not an accident. He asks Bill if he knows Merrill. Merrill could have killed his friend over Rebecca. They fought inside the bar. Bill admits that his son occasionally drank too much. This could have been the reason for the fight. When Gibbs mentions that Merrill’s cell phone could have triggered the fire, Danforth’s father gives him an alibi by saying he was with him at the time of the explosion. Further complicating matters, Tony phones Gibbs to report that Merrill has disappeared since Merrill last saw Gibbs.

Gibbs goes to Director Shepard to summarize the case around a bottle of wine and a “steak au poivre”. Gibbs doesn’t understand what happened. He believes that Merrill’s alibi is a lie, but he is also convinced that Danforth’s grief is real. There is no evidence that Danforth’s son is dead since the DNA does not match. Jenny’s conclusion is that, however it happened, Jethro has to find Merrill. He could find Danforth Junior and understand what happened.

Ducky informs Gibbs that it may take weeks to identify the remains using the dental records. Abby has found Danforth’s prints on Merrill’s prescription bottle and that the content was not expected. It was an anti-rejection drug. Danforth could have had a transplant and the DNA test could have been made against the one of the donor of the organ.

Gibbs meets Danforth Sr. again. The latter admits that his son had a kidney transplanted. The donor was his best friend Kenneth Merrill. This information was kept secret by the family as Marines are not allowed to remain in their Corps if they have had a transplant. Revealing this information could have cost both young men their careers. After his son's death in the car accident, Danforth’s father realized that Merrill’s future with the Marine would be compromised if it was found that he had donated one of his kidneys. The DNA test during the autopsy would have revealed it. The father confessed that he paid the morgue to swap the body of his son with the one of a John Doe and that he was the person responsible for blowing up the ambulance.

Now understanding the case, Gibbs goes as far as helping his family “The Marine Corps” by rearranging evidence to keep everyone happy: the father, the SEC-NAV and his own Director.