Lt. Jonas Cobb is a villain who appears as the main antagonist of Season 8 of NCIS. He is a serial killer known as the "Port-to-Port Killer", and was originally a US sailor. He was hired by the CIA as part of an operation codenamed Frankenstein, which Director Vance supposedly had a part in as well. The operation involved recruiting US sailors to be part of a behavior modification program which would lead to them being able to remain calm and composed in the worst and most extreme of circumstances. The CIA would then use them as assassins on various sides of the globe with them using their cover as Navy sailors. Cobb passed the training and was sent on a first mission to kill an Al-Qaeda operative. He succesfully completed the task but this added to the psychological strain of his training, he believed that he no longer had to respond to authority. Upon returning to the program, the CIA realized that he had broken and decided to send him back to his unit with half a dozen passports with different identites. Cobb later disappeared.

He resurfaced in Rota, Spain as a serial killer and he targeted an enlisted man who came to port. He later killed in different locations such as Japan, Guam and Norfolk before heading to Washington. There, he was met by Gibbs's team and also EJ Barrett's team who had been hunting him since Rota. He was able to keep ahead of them and out of their reach.

He then headed to Hawaii where Barrett and her team followed him. He tried to kill another sailor but was stopped by Trent Kort who lost an eye in the process. He later delivered this eye to DiNozzo through a drink.

The CIA had identified Cobb as the Port to Port killer and had been trying to stop him. Kort claims that the only reason that NCIS hadn't been informed was because it was their mess and they wanted to clean it up.

Cobb began to observe Barrett's team in great detail and even infiltrated the Navy Yard to watch them while all the while setting a trap for them. He killed an NCIS agent and stole his access card and arranged for himself to be seen on a security camera. He later killed Mike Franks as the latter was walking out of Gibbs's house. Cobb was injured in the attack by Franks, but was able to escape before Gibbs arrived. It turned out that he sympathized with Gibbs for being put below Barrett, a similar situation to what Cobb felt, he saw this for himself in the Navy Yard and acted, even though Gibbs realized that Cobb was in his house before killing Gibbs's mentor.

He later ambushed Barrett's team as they were searching the building he had arranged for them to find. He killed Agent Levin (his last murder victim) in the resulting fire fight, wounded Agent Cade severely (but he would later recover), and struggled briefly with Agent Barrett until he managed to overpower her and place a gun to her throat. He wrapped her in plastic wrap and placed her in the trunk of her car, keying in a phone number on the side of the car before leaving (the number belonging to Ziva's newest boyfriend). She was later discovered by Agent Gibbs in Pyramid, and survived the ordeal.

After trapping Ziva and moving her elsewhere, Cobb strangely gave himself up to NCIS without a fight, though he was not done yet. In the resulting interrogation, he ominously relayed to Gibbs a story of a young boy (Cobb) and his horse, who was near death. The boy called his father, a Marine Captain, who, in order to "teach a proper lesson" to his son, did not call a vet to order euthanasia, but instead shot the horse point-blank to "end its misery." Gibbs then asks Cobb if he'd like him to end his misery, probably .

Following the story as a lead, Gibbs goes to Abby who has found traces of certain lineaments on Cobb's pants, lineaments which could be used to sooth aching joints in animals. Gibbs locates the abandoned barn which used to be Cobb's, and discovers a cymbal-playing monkey at the scene, along with a silenced but alive Ziva. She immediately warns the crew that she was a diversion so that Cobb could get what he wants, which is inside of NCIS DC. Cut-over to EJ, Cobb, and Palmer, who all enter (Palmer re-enters it after leaving) the elevator to the basement. Palmer mentions after being questioned by EJ that he is indeed engaged to be married. The elevator opens in the basement and Cobb wastes no time in attacking his restrainer, then holding Palmer hostage and forcing him and EJ to get the van in the basement for a drive.

Gibbs, Vance, and the SECNAV are in pursuit, figuring, correctly, that the Port to Port killer would likely return to where his ordeal began—the building in which Project Frankenstein was conducted—and find him there, "training" three masked individuals (Kort, Palmer, and EJ) by spraying them with a water hose. After a brief encounter with the SECNAV, Cobb unmasks his victims, knocks out Kort with his own gun, and holds EJ by the throat, gun pointed at her. The SECNAV, who arrived first, says that they could speak about it. Gibbs and Vance back him up shortly, telling Cobb to surrender. SECNAV tells the Port to Port killer that he wants to help him.

Cobb pauses, looks up for a moment, then back at the SECNAV. "Liar," he says, but, before he can act, Vance and Gibbs shoot him repeatedly. He falls out of a window onto a sedan parked outside multiple stories lower, dying instantly and ending P2P's criminal career. It appears that he has a grin on his face in the final shot of his body. EJ, Kort, and Palmer survive, as do the ones who went to retrieve them.