In 2 BBY, as part of preparations for the Alliance to Restore the Republic's planned assault on Lothal, the Phoenix Cell, under the command of Phoenix Leader Hera Syndulla, tasked her Spectres team with a critical operation: acquiring Imperial codes. Their target was the Imperial Security Bureau station situated on Killun 71. This initial skirmish in the Galactic Civil War involved Wedge Antilles, cleverly disguised as an Imperial pilot, piloting a captured Sentinel-class landing craft to Killun Station. On board were the droids AP-5 and Chopper, whose mission was to infiltrate the station and extract the necessary codes. AP-5, due to his prior service within the Imperial ranks, navigated the security protocols and successfully retrieved the codes without incident. However, Chopper, lacking proper authorization, had previously drawn the attention of Grand Admiral Thrawn as a potential rebel operative due to his involvement in past operations. Consequently, LT-319, an officer within the Imperial Information Office, identified the astromech droid from his listener ship and seized control of Chopper as he attempted to pilfer the clearance codes from a terminal within the station. Rather than alerting the station's personnel, LT-319 opted to exploit Chopper in an attempt to uncover the location of Phoenix Squadron's base. AP-5 swiftly detected the anomaly in Chopper's behavior and attempted to alert Wedge while en route to their rendezvous point with the Ghost. Yet, the inventory droid's overbearing persistence, which included pursuing Wedge into the refresher, coupled with Chopper's uncharacteristic friendliness towards Wedge, led the Lieutenant to believe the droids were "engaged in some sort of espionage game."
Upon their return to the Ghost, Hera quickly discerned that something was amiss with her droid companion. LT-319 strategically used Chopper to ensnare the Rebels within the freighter's cargo bay, trapping them before deploying a data spike to siphon the base's coordinates from the ship's navicomputer. An attempt to eject the Spectres, AP-5, and Wedge into the vacuum of space was foiled, prompting Hera to dispatch AP-5 to the Ghost's exterior to access controls that would unlock the cockpit hatch. He succeeded, but his actions were observed by the mind-controlled Chopper, who was then ordered to eliminate the protocol droid, knocking him off the Ghost's hull and leaving him stranded in space. Upon Chopper's return, he was ambushed by the Spectres, and Hera seized the opportunity to reverse the control signal using the data spike, remotely detonating LT-319's surveillance vessel. Subsequently, they returned to rescue AP-5. Given that the controller had not notified any other Imperial authorities about the theft, the Rebels' mission was deemed a success.
Chopper, a C1-series astromech droid, was an integral part of the rebel cell known as the Spectres, operating under the banner of the Alliance to Restore the Republic. He maintained a close friendship with AP-5, a former Imperial RA-7 inventory droid who had defected to the rebellion after assisting in the discovery of a safe base located on the planet Atollon. Together, the two droids participated in numerous missions for the rebellion, even playing a crucial role in thwarting an Imperial effort to pinpoint Chopper Base's location.
Within the rebellion, Chopper frequently undertook clandestine missions within Imperial facilities and starships. During these operations, he was repainted in black and red to resemble Imperial astromech droids. During one particular mission to infiltrate the Imperial Armory Complex on Lothal, Chopper was identified by the Imperial strategist Grand Admiral Thrawn as an unregistered C1 astromech droid. Thrawn meticulously documented the rebel droid's appearances for distribution among Imperial forces.
In preparation for Phoenix Cell's impending attack on Lothal, Phoenix Leader Hera Syndulla dispatched AP-5, Chopper, and Lieutenant Wedge Antilles on a mission to pilfer a set of clearance codes from the Imperial Security Bureau's base on Killun 71. Hera entrusted AP-5 with leadership of the mission, tasking him and Chopper with downloading the codes from the station's central computer. The Rebels successfully gained permission to land at the base under the guise of delivering two service droids.
AP-5 managed to bypass the base's security sensors without issue. However, Chopper triggered an alert during the scanning process, which went unnoticed by the stormtroopers on guard but was detected by the crew of an Imperial Information Office IGV-55 surveillance vessel stationed in deep space. The ship's controller, LT-319, recognized Chopper as the unregistered rebel droid mentioned in Thrawn's manual on Phoenix Squadron. While one of his subordinates, 3-9, advocated for informing the ISB station, the Imperial controller decided to exploit Chopper's presence to locate Phoenix Squadron's hidden base.
Chopper attempted to acquire the codes by plugging into a network terminal, despite AP-5's warning that the most up-to-date codes would not be accessible there. The exasperated protocol droid proceeded to the main control room to retrieve the codes himself, leaving Chopper at the terminal. LT-319 and his crew seized the opportunity to hijack the rebel astromech droid, searching for the base's coordinates. After determining that the coordinates were not stored in Chopper's memory, the controller took direct control of Chopper, intending to use him to lead them to the rebel base. Outside the control room, AP-5 encountered an ISB droid, PZ-7, but managed to dismiss her by claiming he was her replacement and that she had been reassigned to the cargo platform. After AP-5 successfully stole the clearance codes, he discovered the Imperial-controlled Chopper still at the terminal and taunted him about his own superior effectiveness before departing for the shuttle.
Upon boarding the shuttle, AP-5 urged Wedge to depart immediately. When Wedge inquired about Chopper's whereabouts, the hijacked droid boarded their stolen Sentinel-class landing craft. While AP-5 was suspicious of Chopper's altered behavior, Wedge noticed nothing unusual and even welcomed Chopper's newfound respectful demeanor. AP-5 attempted to caution Wedge, even following him into the shuttle's refresher to speak privately. The controller, observing through Chopper, witnessed Wedge ejecting AP-5 from the refresher and concluded that the inventory droid had undermined his own credibility.
After the shuttle docked with the Ghost, AP-5 handed Hera the disc containing the clearance codes. Wedge recounted the droids' behavior during the flight, which amused Ezra Bridger and Garazeb Orrelios. The Imperial-hijacked Chopper then proceeded to the Ghost's cockpit and began downloading the ship's hyperspace logs. By this point, AP-5 had successfully alerted Hera to Chopper's suspicious actions. Hera confronted Chopper, who claimed that files had been wiped on Killun, and reminded the droid that she routinely cleared his data after every hyperspace jump for security purposes. After she left to consult with the rest of the crew, Chopper disengaged the ship from hyperspace and hid himself away. When Hera contacted him, demanding an explanation, Chopper claimed to be in the engine room, reporting an issue with the hyperdrive. Hera gathered the other crew members, all of whom were now convinced that something was wrong with Chopper, to locate him. The hijacked droid then locked the rebels inside the Ghost's cargo hold and jettisoned the stolen Imperial shuttle, flying away.
Having secured the stolen Imperial clearance codes, Chopper proceeded to install a data spike into the Ghost's navigation computer. This allowed the Imperials to remotely download the ship's travel logs. In an attempt to eliminate the rebels, the controller ordered Chopper to open the cargo hold to space. However, Hera managed to manually override the door. Unable to open the cockpit hatch from the inside, she instructed AP-5 to open it from the outside. While traversing the ship's hull, AP-5 was spotted by Chopper and the controller through the cockpit window.
Determined to neutralize the rogue RA-7 droid, the Imperial controller dispatched Chopper to confront AP-5. Chopper successfully pushed AP-5 into space, moments after the inventory droid had unlocked the hatch. Upon Chopper's return, Zeb incapacitated him with his bo-rifle. Hera then connected Chopper to the Ghost's systems and decided to send a stern warning to the Imperial slicers. Utilizing Chopper's visual processor, Hera delivered a threat to LT-319 and transmitted a data overload that disabled the Gozanti cruiser's systems, resulting in its destruction and the demise of its entire Imperial crew.
Following the elimination of the Imperial threat, Hera reprogrammed Chopper. After the Spectres reconciled with Chopper, they rescued AP-5 from the void of space. Despite the hijacking and subsequent reprogramming of Chopper, the Spectres managed to retain the clearance codes they had stolen from Killun 71. The destruction of the Imperial communications cruiser also prevented news of the rebel theft from spreading, allowing Phoenix Squadron to continue their preparations for the strike on Lothal.
The mission to Killun Station was first depicted in the Star Wars Rebels season three episode "Double Agent Droid," which premiered on Disney XD on March 11, 2017.