In the mid-2360s, Jarok's home on Romulus was located near the Apnex Sea, where he lived with his wife and daughter. The experience of the birth of his child changed his character and made him reconsider his views on war. (TNG: "The Defector")
In 2366, with Romulan-Federation tensions once again on the rise, Jarok became convinced that another war would destroy the Empire. For months, he argued his viewpoint futilely with the Romulan High Command, until he was censured and dismissed to command a distant sector. His loyalty now in doubt, Jarok became the target of a deliberate misinformation program by Romulan Command. He was sent false communiques, records, and orders concerning the establishment of a secret base on Nelvana III, in the Neutral Zone, in preparation for a massive invasion of Federation space.
Not knowing the information he was being fed was false, he thought his people were on the brink of another war and, unwilling to allow a war that would devastate both sides, Jarok stole a Romulan scout ship and crossed the Neutral Zone, seeking refuge aboard the USS Enterprise-D. Claiming himself to be "Sublieutenant Setal," a low-ranking logistics officer, he informed Captain Jean-Luc Picard of the offensive. However, Picard was skeptical, given Jarok's reluctance to reveal any Romulan military intelligence beyond the build-up at Nelvana III, which included the self-destruction of his ship. Only after Jarok revealed his true identity and provided the disposition of the Romulan fleet along the Neutral Zone, did Picard order the Enterprise-D to Nelvana III.
When the Enterprise-D reached Nelvana III, they found no evidence of a base whatsoever – only a probe designed to send out a subspace signal and ionization disturbances. Realizing that they had fallen into a trap, the Enterprise-D attempted to retreat, only to be stopped by two D'deridex-class Romulan warbirds commanded by Tomalak. Tomalak revealed the deception, and announced his intention to take the Enterprise-D back to Romulus as a trophy.
Fortunately, Picard had planned ahead and arranged for substantial Klingon forces to follow them under cloak. Suddenly finding himself outgunned, Tomalak withdrew. Jarok, realizing that he had sacrificed everything for nothing, committed suicide by ingesting a felodesine chip. He left only a letter for his family, and the memory of his courage. (TNG: "The Defector")
"I am not a traitor. All you can see is the opportunity to exploit me. The Federation credo, exploitation. You couldn't get aboard my ship fast enough. Strip it down. What secrets might it reveal that we can use? You're a short-sighted people. Can't you understand? I came to stop a war."
- - Alidar Jarok
"What a fool I've been. To come looking for courage in a lair of cowards."
- - Alidar Jarok, during questioning by William Riker
"Am I correct in assuming you regret your decision to come here?"
"What I did had to be done. But to never again see the firefalls of Gath Gal'thong, and the spires of my home as they rise above the Apnex Sea at dawn. It's a bitter thing to be exiled from your home."
- - Data and Alidar Jarok
"I cannot betray my people."
"You've already betrayed your people, Admiral. You've made your choices, sir. You're a traitor. Now, if the bitter taste of that is unpalatable to you, I am truly sorry. But I will not risk the lives of my crew because you think you can dance on the edge of the Neutral Zone. You've crossed over, Admiral. You make yourself comfortable with that."
- - Alidar Jarok and Jean-Luc Picard
"There comes a time in a man's life that you cannot know. When he looks down at the first smile of his baby girl and realizes he must change the world for her... for all children. It is for her that I am here. Not to destroy the Romulan Empire, but to save it. For months, I tried desperately to persuade the High Command that another war would destroy the Empire. They got tired of my arguments. Finally, I was censured, sent off to command some distant sector. This was my only recourse. I will never see my child smile again. She will grow up believing that her father is a traitor. But she will grow up. If you act, Picard. If we stop the war before it begins."
- - Alidar Jarok, to Jean-Luc Picard
"Nelvana III, Admiral! No base, no weapons, no sign of any life at all!"
"But I…I SAW the tactical communiques, the records, timetables for completion. An entire legion was assigned to this section!"
"Is it possible they could have been feeding you disinformation? You said that you had been censured, reassigned four months ago, they knew of your dissatisfaction. Could all this have been to test your loyalty?"
"No! No, it's impossible!"
"They let you escape, with an arsenal of worthless secrets. What other explanation is there? "
- - Jean-Luc Picard and Alidar Jarok
"A letter to his wife and daughter."
"Sir, he must have known it would be impossible for us to deliver this."
"Today, perhaps. But if there are others with the courage of Admiral Jarok, we may hope to see a day of peace when we can take his letter home."
- - William Riker, Data, and Jean-Luc Picard, after discovering Jarok has committed suicide.
Whereas Jarok uses the name "Setal" as a pseudonym in the final version of "The Defector", that was originally to have actually been his name. At the same early stage in the episode's development, Setal was envisaged as being "about 38 human years of age." He was described that way in an early story outline for "The Defector". Also in that document, the first things he did upon being beamed aboard the Enterprise were requesting asylum and then collapsing in the transporter room, whereas, in the final version of the episode, he comes aboard the ship demanding to speak with the ship's captain, believing he has information which is urgent and vital to the Enterprise-D crew.
The original script for "The Defector" had Jarok becoming romantically involved with Beverly Crusher but this was later dropped. (Star Trek: The Next Generation Companion (1st ed., p. 111))
The final script gave the pronunciation of Jarok's name as "JAH-rock" and went on to describe the character as "a Romulan male of about fifty (human) years of age." 
Alidar Jarok appears in a number of non-canon works.
In The Lost Era novel Catalyst of Sorrows, Jarok and his long-time friend Tal visit a jewelers in order to find a naming day gift in 2360. While looking for the gift, the two officers speak about the futility of war and a hope for peace. These words are heard by a young spy named Zetha, who is apprenticing at the jewelers by the arrangement of her "Lord", because military officers often frequent the shop. Despite the content of Jarok's conversation, Zetha does not report it to the "Lord".
Jarok is also mentioned in A Singular Destiny, in which the Federation character of Professor Sonek Pran writes a monograph on Admiral Jarok in 2364, concluding he is a patriot of the highest order. One of the sources Pran uses is an officially sanctioned biography of Jarok, which had been published earlier, and smuggled into the Federation.
In The Sky's the Limit short story "Suicide Note", Captain Picard hand delivers Jarok's suicide note to Romulus in the year 2374, after the Federation and the Romulan Empire have become reluctant allies in the Dominion War. In the story, Jarok's wife is named Ai'lara and his daughter is named Tiaru, aged eleven at the time of Picard's visit (making her three years old at the time of Jarok's departure). His note takes the form of a video recording, sending his love to his family and warning his daughter against the evils of paranoia and militarism, that have already damaged the Romulan Empire so much. In particular, he was appalled to learn that Romulan deep-space probes had gathered early warnings of the imminent invasion of the Alpha Quadrant by the Borg, but the Continuing Committee brushed these aside and remained focused on the ongoing conflicts with the Federation and the Klingon Empire. He also stuns them by revealing that they have been under the secret protection of General Koval, one of Jarok's sympathizers within the Tal Shiar. The message also gives Picard the secret codes needed to reactivate Koval as an agent for Federation Intelligence. After leaving the Jarok residence, Picard signals Will Riker in the capital city, and is pleasantly surprised to learn that Koval is the recently-promoted Vice Chairman of the Tal Shiar (replacing the recently-deceased Senator Vreenak).
Jarok's daughter Tiaru is also prominently featured in the video game Star Trek Online. By 2409-2410, she is the commander of the warbird Lleiset, the flagship of the Romulan Republic, a breakaway faction from the Romulan Empire led by unificationist leader D'Tan.
Jarok also has a card in the Star Trek Customizable Card Game, which describes him as a "Conscientious Admiral".