Mission to recover Imperial codes

In the year 2 BBY, the Phoenix Squadron dispatched Phoenix Leader Hera Syndulla and her Spectres crew on a mission to steal the clearance codes from the Imperial station on Killun 71 for the Alliance to Restore the Republic's attack on Lothal.

Prelude
The C1-series astromech droid known as Chopper was a member of the rebel cell known as the Spectres, which was part of the Alliance to Restore the Republic. . He was a friend of the former Imperial RA-7 inventory droid AP-5, who had joined the rebellion after helping the rebels to locate a safe base on the planet Atollon. Together, the two droids undertook several missions on behalf of the rebellion and even helped foil an Imperial operation to locate Chopper Base.

As part of the rebellion, Chopper undertook numerous undercover missions into Imperial installations and starships. On numerous occasions, he was painted in black and red colors to mimic Imperial astromech droids. During a mission to infiltrate the Imperial Armory Complex on Lothal, Chopper was detected by the Imperial tactician Grand Admiral Thrawn, who identified him as an unregistered C1 astromech droid. Thrawn kept a catalogue of the rebel droid's sightings for Imperial forces.

Killun Station
While preparing for the Phoenix Squadron's attack on Lothal, Phoenix Leader Hera Syndulla dispatched AP-5, Chopper, Lieutenant Wedge Antilles on a mission to steal a set of clearance codes from the Imperial Security Bureau's Killun Station on Killun 71. AP-5 was tasked with downloading the codes from the station's central computer while Chopper and Wedge provided back-up. The rebels managed to obtain permission to land at the base under the pretext of delivering two infantry droids.

AP-5 and Chopper managed to pass through the base's sensors without any problems. However, Chopper's presence was detected by the crew of an Imperial Information Office IGV-55 surveillance vessel in deep space. The ship's controller LT-319 recognizes Chopper as the unregistered rebel droid that Thrawn had referenced on his manual on the Phoenix Squadron. While one of his subordinates 3-9 wanted to inform the ISB station, the Imperial controller decided to use Chopper to find the Phoenix Squadron's hidden base.

While Chopper was bidding his time by plugging a network terminal, LT-319 and his crew took the opportunity to hijack the rebel astromech droid and reprogram him into an Imperial droid. After AP-5 stole the clearance codes, the Imperial-controlled Chopper followed them aboard their stolen Sentinel-class landing craft. While AP-5 was suspicious about Chopper's change in behavior, Wedge did not notice anything untoward and even welcome Chopper's more respectful attitude.

Spy games
The Imperial-hijacked Chopper then made his way to the Ghost cockpit and proceeded to download the ship's hyperspace logs. By that time, AP-5 had managed to warn Hera about Chopper's suspicious behavior. Hera managed to stop Chopper and reminded the droid that she cleared his data after every hyperspace jump for security reasons. Chopper managed to slip away into the engine chamber and deactivate the hyperdrive while Hera roused the other crew. When the other rebels pursued him, Chopper locked them in the cargo bay.

After recovering the stolen Imperial clearance codes, Chopper began to install a data spike into the Ghost navigation computer. This enabled the Imperials to begin remotely downloading the ship's travel logs. In an attempt to kill the rebels, the controller ordered Chopper to open the cargo hatch to space. However, Hera managed to manually shut the door. Unable to open the cargo bay from inside, she sent AP-5 to open the hatch from outside.

Determined to eliminate the rogue RA-7 droid, the Imperial controller sent Chopper to fight off AP-5. Chopper managed to push AP-5 into space but not before the droid unlocked the cargo hatch. After Chopper returned, he was stunned by Garazeb Orrelios' bo-rifle. Hera then linked Chopper to the Ghost systems and decided to send a strong message to the Imperial slicers. Using Chopper's visual processor, Hera issued a threat to LT-319 and transmitted a data overload that disabled the Gozanti cruiser's systems, causing it to explode and killing all of the Imperial crew.

Aftermath
"No one messes with my droid."

- Hera Syndulla

After eliminating the Imperial threat, Hera reprogrammed Chopper. After the Spectres reconciled with Chopper, they rescued AP-5 from space. Despite the hijacking and reprogramming of Chopper, the Spectres managed to keep the clearance codes they had stolen from Killun 71. The destruction of the Imperial communications cruiser also concealed word of the rebel theft and allowed Phoenix Squadron to continue making preparations for their strike on Lothal.

Behind the scenes
The first appeared in the Star Wars Rebels Season Three episode "Double Agent Droid," which premiered on Disney XD on March 11, 2017.