EXD-9's infiltration of Chopper Base

In 2 BBY, the Imperial Grand Admiral Thrawn began deploying E-XD infiltrator droids to several Outer Rim planets. One of them, EXD-9 landed on Atollon, home of the Phoenix rebel cell's Chopper Base. EXD-9 was soon ambushed by several krykna spiders and knocked out. The Infiltrator droid was later found by Captain Garazeb Orrelios and Chopper, who brought the droid back to their base. Mistaking EXD-9 for a protocol droid, they assigned him to help AP-5 manage the base depot's inventory. After learning that EXD-9 was an Imperial Infiltrator droid, Rex and Chopper and AP-5 joined forces to overpower and deactivate the droid. To prevent the Empire from discovering the location of Chopper Base, the rebels sent him back to his Star Destroyer with his proton torpedo warhead activated. EXD-9 subsequently managed to destroy the Star Destroyer but Thrawn managed to narrow down his search for the rebel base to 94 systems.

Prelude
In 2 BBY, Grand Admiral Thrawn began deploying E-XD infiltrator droids to sweep the Outer Rim Territories for rebel bases. Against Imperial mandate, these droids were fitted with a proton warhead that would self-destruct in the event that they were captured. One of these droids, EXD-9, landed on the planet Atollon, which was the location of the Phoenix rebel cell's Chopper Base. After landing, EXD-9 was attacked by two spidery krykna and temporarily disabled.

EXD-9's arrival coincided with most of the Phoenix rebels including most of the Spectres cell departing on a training exercise. The Lasat warrior Garazeb Orrelios was tasked with overseeing the base's security. The rest of the stay-behind party included the C1-series astromech droid Chopper, the RA-7 protocol droid AP-5, and a communications officer stationed at the command center. After the comms officer reported that a sensor beacon in Sector 6 had been knocked out by meteors, Zeb and Chopper went to investigate.

Intruder alert
Upon arriving, Zeb and Chopper found EXD-9, whom they mistook for a protocol droid. Taking pity on the stray droid, Zeb brought EXD-9 back to Chopper Base. There, AP-5 recharged the droid, who began scanning the depot. Believing that EXD-9 would make a good inventory droid, AP-5 hired the protocol droid as his assistant. Later, Zeb received a transmission from Fulcrum warning that the Empire had deployed Infiltrator droids and that the Empire had lost contact with one of them. After viewing the schematics, Zeb realized that EXD-9 was an Infiltrator Droid. Fulcrum was actually the disillusioned Imperial Security Bureau Agent Kallus, who had been aiding the rebellion by providing them with intelligence on rebel movements since his encounter with Zeb on Bahryn.

Zeb returned to the depot just in time to warn AP-5 and Chopper before EXD-9 transformed into his attack mode. The Infiltrator droid attacked the rebels and fought with Zeb. During the struggle, Zeb managed to sever the droid's left hand. Damaged, EXD-9 retreated to recharge and repair himself. He stumbled upon another astromech droid and a GNK power droid whom he destroyed. He then drained their power and cannibalized their parts to recharge and repair himself. In response, Zeb and the droids devised a plan to lure EXD-9 out of hiding so that they could disable him.

Return to sender
Chopper managed to lure EXD-9 out of hiding and Zeb attacked the droid. Following a fierce struggle, Zeb managed to pin down the Infiltrator droid long enough for AP-5 to disable him. When the droid began activating its proton warhead, Chopper managed to freeze the timer before it could initiate detonation. The rebels then faced a dilemma. Knowing that the Empire would search for the droid, they also had to protect the secrecy of Chopper Base. After some discussion, Zeb and the droids decided to reactivate EXD-9 to its disarmed mode and return the droid to its base with the proton warhead. AP-5 also set EXD-9 to explode once he had reconnected with the Imperial network.

After reactivating EXD-9, Zeb, AP-5, and Chopper allowed the Infiltrator droid to walk back to his pod. Tailing EXD-9, Zeb shot out the pod's long-range transmitter. This forced EXD-9 to take his pod back to his Imperial Star Destroyer. Upon arriving, two Imperial Navy technicians connected EXD-9 back to the Imperial network. This caused the Infiltrator droid to initiate his proton bomb countdown and detonate, which also caused the other Infiltrator droids to explode as well. The subsequent chain reaction of powerful explosives tore the Star Destroyer apart. The explosion was watched by Agent Kallus and several Imperial crew aboard a nearby Star Destroyer. While expressing shock and anger publicly, Kallus privately smiled.

Aftermath
The following day, Zeb contacted Hera Syndulla and the other Spectres to inform her about the incident but that they had things under control. The Spectres later attended a briefing at the Chopper Base command center where they received a message from Fulcrum congratulating them for reprogramming the Infiltrator droid that he had warned them about and setting it to self-destruct. AP-5 tried to tell the other Spectres the whole story but Zeb playfully shut him up.

Grand Admiral Thrawn learned about EXD-9's role in destruction of the Star Destroyer from Kallus and complimented the ingenuity of the rebel plan. Despite the setback, Thrawn was in good spirits because he had narrowed his search for the Phoenix rebel base down to 94 systems. He vowed to crush the rebellion once and for all.

Behind the scenes
EXD-9's infiltration of Chopper Base served as the main plot of the Star Wars Rebels episode "Warhead", which first premiered on January 14, 2017. The plot background and details were further expanded in a Rebels Recon video hosted by Andi Gutierrez and Pablo Hidalgo.