The rebel cell known as Phoenix Cell found themselves under Admiral Kassius Konstantine's assault. They were awaiting a fuel delivery from the Ghost's crew before initiating a hyperspace jump to the Yost system, their intended location for establishing a new base.
However, they soon discovered that the Yost system was under Imperial control, turning their destination into an ambush. With the assistance of AP-5, an Imperial inventory droid, the rebel astromech droid Chopper identified an alternative location: Atollon. Evading the attack, the rebels successfully jumped to Atollon.
Following the theft of the Imperial fighter carrier above the planet Ryloth, Phoenix Cell continued its search for a permanent base of operations. By analyzing various star charts from the Old Republic and the Lasat, the rebels, influenced by Ketsu Onyo, a friend of Sabine Wren, decided on Berzite's moon within the Yost system. However, the rebel fighter carrier lacked the necessary fuel for the journey.
As a result, the Spectres embarked on a mission to acquire fuel from the Imperial depot at Horizon Base. The rebels succeeded in their fuel heist, but the astromech droid Chopper became separated from the group when he was sidetracked by a droid leg. Despite being pursued by Imperial stormtroopers, Chopper managed to board an Imperial cargo ship known as Cargo transport 241. There, he formed an unexpected alliance with AP-5, an Imperial inventory droid with a history dating back to the Clone Wars. Together, the two droids incapacitated the ship's captain and took control of the vessel, ejecting the cargo compartment and the stormtrooper garrison into space.
Upon the Spectres' return to the rebel fleet, they found themselves under attack by an Imperial force. Admiral Kassius Konstantine, recognizing the rebel starship Ghost as the same vessel that had raided Horizon Base, deduced that it was carrying fuel for the rebels and directed his ships to concentrate their fire on the Ghost. While the Ghost was pursued by TIE fighters, Ketsu intervened in her starship Shadow Caster to assist the rebels. After evading the Imperial ships, the Ghost successfully landed within the rebel fighter carrier.
As Ezra Bridger and Garazeb Orrelios unloaded the fuel, Chopper contacted the Ghost, informing Captain Hera Syndulla about his acquisition of an Imperial cargo ship. When Hera mentioned the rebel fleet's impending departure for the Yost system, AP-5 promptly warned her of an Imperial ambush awaiting them there. After Chopper vouched for AP-5's credibility, Hera agreed to trust the former Imperial inventory droid. By cross-referencing data from both Chopper and Imperial sources, AP-5 identified Atollon as a safe haven devoid of Imperial presence.
Acting on AP-5's information, Hera alerted Commander Jun Sato and the other rebels to the dangers of the Yost system. Before AP-5 could transmit the new hyperspace coordinates for Atollon, he and Chopper were confronted by the Imperial ship captain, who had regained consciousness. During the ensuing struggle, Chopper managed to subdue the Imperial, but not before the captain shot AP-5 twice with his blaster, causing significant damage to the droid. Chopper then completed AP-5's task by transmitting the remaining coordinates to Hera, who relayed them to Commander Sato. Thanks to AP-5's information, the Phoenix rebels managed to avoid the Imperial trap and reach Atollon.
After the rebel fleet arrived at Atollon, Chopper and AP-5 rejoined them. Following repairs by Sabine, AP-5, encouraged by Chopper, decided to join the rebellion. While the rebel fleet remained in orbit, rebel forces under Captain Rex began constructing a base on Atollon. This new base was named "Chopper Base" in honor of Chopper. Rebel efforts to establish the base faced a challenge in the form of predatory spider-like creatures known as krykna. The krykna devoured Lieutenant and kidnapped Rex, who was later rescued by the Spectres. Determined to maintain the base, the rebels erected a perimeter fence using sensor markers, a security technology that irritated the krykna and kept them at bay.
The attack on the rebel fleet forms the climax of the Star Wars Rebels episode "The Forgotten Droid," which premiered on Disney XD on March 16, 2016. This episode introduced new characters and plot elements, including the inventory droid AP-5 (voiced by Stephen Stanton), the Imperial cargo ship captain (James Adomian), and the planet Atollon.