In the year 3643 BBY, amidst the Cold War raging between the Galactic Republic and the Sith Empire, a young Jedi Knight, accompanied by Kira Carsen, his Padawan, embarked on a quest. Their destination: the heart of Justicar territory, the base of operations for the paramilitary Justicars' Brigade on Coruscant, the Republic's capital planet. The Jedi's objective was to recover the Planet Prison, a Republic superweapon pilfered by scientists working on the project, all at the instigation of their superior, Doctor Tarnis. Unbeknownst to these scientists, Tarnis was secretly a Sith Lord.
By the time the Knight and Carsen arrived at the Justicars' Tower, where they had pinpointed an earlier transmission from Tarnis, the Sith had already killed the research team and absconded with the Planet Prison. Jonkan, the sole survivor, mortally wounded, assisted the two Jedi in locating the Planet Prison deep within the Jedi Temple. Subsequently, the Jedi pursued Tarnis, ultimately defeating him in a duel and deactivating the weapon, thus foiling the Sith's scheme to deploy it against the Republic capital.
Back in 3643 BBY, as the Cold War persisted between the Galactic Republic and the Sith Empire, both the Republic and the Jedi Order discovered that Doctor Eli Tarnis, the leading scientist overseeing the Republic's Planet Prison superweapon initiative, was, in reality, a Sith Lord. Tarnis's true allegiance was revealed when he orchestrated the theft of files pertaining to the Republic superweapon initiative and aligned himself with the Black Sun criminal organization. Tarnis then contacted his research team, who were unaware of his true nature, and urged them to deliver the Planet Prison prototype to him, falsely claiming that the Jedi intended to terminate the project. Trusting Tarnis, the five scientists secured the prototype and transported it to Tarnis, who was located deep within Justicar territory, the domain of the paramilitary and Imperial-sympathetic Justicars' Brigade.
A young Jedi Knight, along with the astromech droid T7-O1, had initiated a rescue mission within the territory controlled by Black Sun, prompted by Tarnis's staged kidnapping from the Senate Building. However, instead of locating the scientist, the Jedi Knight uncovered Tarnis's deception and his true identity as a Sith. Upon returning to the Senate Building, the Knight was dispatched by General Var Suthra of the Republic Military to pursue Tarnis and the stolen Planet Prison to the Justicars' Sector, the source of Tarnis's transmission to the Knight. The Jedi Padawan Kira Carsen accompanied the Knight, anticipating a potential encounter with the Sith Lord during their search, and together they departed for the Justicars' Sector.
As the two Jedi traveled via air taxi toward the Justicars' Sector, Tarnis convened with his research team within the Justicars' Tower, the Brigade's prison and interrogation facility. After securing the Planet Prison, Tarnis betrayed his team, revealing his Sith identity and subsequently murdering them using Force lightning. However, one of the scientists, a man named Jonkan, sustained only grave injuries and witnessed Tarnis departing with the Planet Prison prototype.
The Knight and Carsen were compelled to battle their way through hostile Justicars to reach their destination. However, by the time they arrived, Tarnis had already vacated the area, and the two Jedi discovered only the bodies of the research team and the dying Jonkan. Aware of his critical condition, Jonkan dismissed the Jedi's offers of assistance and quickly recounted the events that had transpired. In his final moments, Jonkan explained how the Knight and Carsen could track the Planet Prison: The device generated substantial heat and required time to reach full charge, providing them with an opportunity to locate and destroy the weapon before it could be fired. Urging the Knight to save Coruscant, Jonkan uttered his final words and passed away.
Utilizing Jonkan's information, Carsen tracked the Planet Prison's location using the room's computers. Tarnis had transported the weapon to the ruins of the Jedi Temple as a final insult to the Jedi Order. With no time to spare, the two Jedi sped through the Works, Coruscant's industrial and infrastructural district, toward the Temple for their final confrontation with Tarnis. The pair located Tarnis amidst the Temple ruins and defeated him in a duel, although Tarnis's demise incurred the wrath of his father, Darth Angral.
The mission to the Justicar's Sector was designed for Star Wars: The Old Republic, a massively multiplayer online role-playing game launched by BioWare on December 20, 2011. It appears within the Jedi Knight-class storyline as the mission "Tracking Down the Traitor," where the player pursues the Sith Lord Tarnis to the Justicars' Sector in an attempt to locate the Planet Prison superweapon. While the mission lacks alignment choices, it is significant as the initial mission in which the player acquires Kira Carsen as a companion. She serves as a guest companion for this mission and the subsequent one, meaning she cannot be assigned to crew skill missions.