Retired Navy Senior Chief Petty Officer Diego De La Rosa, now a deep sea diver, is in the decompression chamber aboard the DSV Brandywine when he gets violent and almost kills his dive mates. In the struggle, a spark goes off. The dive team is forced to put on air masks, but De La Rosa dies.

The control panel team watches the whole thing and Bob Mugford, a former Navy man, contacts NCIS in the "Decompressed" episode to get justice for his friend.

Gibbs, McGee and Ducky head out to the ship, but when Ducky finds that the body is in a decompression chamber and needs to be there for four days, he says it is useless. The process of decompression will destroy any evidence of how De La Rosa died forever.

Then Ducky gets the idea of doing a "remote autopsy." He has the three remaining divers collect evidence: blood samples, urine and nail scrapings. They pass the autopsy kit through the airlock used to serve them food.

Back in D.C., Bishop goes to see Mrs. De la Rosa, but when she finds out that her husband died in the chamber, not in the ocean, she suspiciously cuts the interview short. Gibbs checks on the breathing apparatus, but discovers that all of the breathing masks are connected to the same oxygen source,ruling out one possible murder weapon.

DiNozzo investigates and tells Gibbs that the remaining three divers each have a secret: Sam Harper has a credit history so bad that he lost his house and car. Meredith Ragen has a dishonorable discharge but her file is sealed. And Jalen Washington made over 20 calls to a prepaid burner phone. Even more suspicious is the fact that the ship is moored more than six miles away from the pipeline that they were allegedly working on.

Gibbs confronts the divers, telling them they lied to a federal agent, and offering a get out of jail card to the first person who comes clean. Just then Jerry Grossman, the attorney for Olympus Underwater Research, shows up with non-disclosure agreements that the entire crew has signed. He says no one can talk to NCIS or they will be sued.

Harper reveals to McGee that Washington was doing a little night diving with Mrs. De La Rosa and speculates that Washington committed the murder to eliminate the competition. Back in D.C., Gibbs goes to see Mrs. De la Rosa. She tells him she was unhappy, but she would never leave her husband due to her Catholic faith. She is shocked at the idea that Washington might have killed her husband.

With the samples from the "remote autopsy," Abby is able to figure out the cause of De La Rosa's death. The toxicology report shows he was high on pharmaceutical cocaine mixed with significant levels of hydrochloride, which turns powder into liquid, as well as red dye. But that isn't what killed him. De La Rosa died from asphyxiation, but didn't choke. He was breathing inert gas. Abby shows Gibbs how someone could tamper with De La Rosa's air hose without anyone knowing, which means one of the three divers killed him.

Tony watches the video from the chamber over and over to see if he can figure out who tampered with the air hose, to no avail. Gibbs gets upset, saying, "There are only three suspects and they are locked in a metal tube. How hard can it be?"

Gibbs is back aboard the ship and tells the captain that De La Rosa was definiately murdered. Grossman threatens legal action, but the captain says the air hose connection was the cause, then Sam Harper was involved. The captain says a diver nearly died two years before because of a misconnected hose. Harper was his partner.

Then DiNozzo gets Ragen's military file unsealed and learns that she killed someone she thought was an insurgent with her bare hands. The victim turned out to be a translator for U.S. forces.

McGee hacks into the ship and finds out the divers are beta testing a secret project called PPA [Prepositioned Assets], watertight closets that can be filled with weapons and left on the bottom of the sea. Then, when you press a button, they pop up to the surface.

Gibbs meets with Navy Admiral Jorge Morales, who tells him the PPA is a revolutionary idea. It would change how wars are fought, but the containers leak, and weapons and seawater don't mix. 

McGee, Gibbs and Abby discuss what could be put in a PPA that wouldn't be affected by seawater The answer is a sealed vial of cocaine. Normally cocaine is clear, so to prove the vial is sealed, it would need to be colored. The closets would be the perfect hiding spot for drug cartels. So when De La Rosa discovered what was going on, he had to be killed. Giving him cocaine wasn't enough, so the killer cut off his air hose.

Mugford tells Bishop that on the morning De La Rosa died, the ship's control got an alert: One of the divers had opened a PPA.

Two days later when the decompression chamber is open, Gibbs arrests Meredith Ragen. Grossman says he has already filed a grievance to have the case thrown out. Gibbs says he knows it was her because there is a small percentage of people that can't eat anything red. They get rashes from the dye and Ragen has a rash. She had to have touched something, like a vial of red liquid cocaine.

But Ragen also had a partner who supplied the cocaine. Gibbs shows Grossman photos of the lawyer in Cartagena meeting with the Suarez drug cartel. Case closed.

Then, the NCIS team celebrates Valentine's Day.

'NCIS' airs Tuesdays on CBS.