They soon learned, however, that the Yost system was under Imperial control and the rebels would be jumping into a trap. With the help of an Imperial inventory droid named AP-5, the rebel astromech droid Chopper was able to discover a new location: Atollon. The rebels jumped away from the attack and arrived safely in Atollon.
Following the theft of the Imperial fighter carrier above the planet Ryloth, the Phoenix Cell continued their search for a permanent rebel base. By cross-referencing several Old Republic and Lasat star maps, the rebels had decided to settle on Berzite's moon in the Yost system at the urging of Ketsu Onyo; Sabine Wren's friend. However, the rebel fighter carrier lacked sufficient fuel to travel to the Yost system.
Consequently, the Spectres were dispatched on a mission to steal fuel from the Imperial depot at Horizon Base. The rebels managed to steal the fuel but the astromech droid Chopper was left stranded after he was distracted by a new droid leg. Despite being pursued by Imperial stormtroopers, Chopper managed to stow aboard an Imperial cargo ship called Cargo transport 241. There, he struck an unlikely friendship with an Imperial inventory droid named AP-5, who had been a veteran of the Clone Wars. Working together, the two droids knocked out the ship's captain and seized control of the vessel. They also jettisoned the ship's cargo compartment and the stormtrooper garrison in space.
The Ghost landing within Phoenix Nest.
As soon as the Spectres returned to the rebel fleet, they discovered that it was under attack by an Imperial fleet. Recognizing the rebel starship Ghost as the same vessel that had raided the Imperial depot at Horizon Base, Admiral Kassius Konstantine realized that the Ghost was carrying fuel for the rebels and ordered his ships to focus their fire on the Ghost. The Ghost was pursued by several TIE fighters but Ketsu came to the rebels' aid in her starship Shadow Caster. After breaking through the Imperial ships, the Ghost managed to land inside the rebel fighter carrier.
While Ezra Bridger and Garazeb Orrelios were unloading fuel, Chopper managed to contact the Ghost and informed Captain Hera Syndulla that he had managed to steal an Imperial cargo ship. When Hera mentioned that the rebel fleet was preparing to depart for the Yost system, AP-5 quickly warned the rebel captain that an Imperial fleet was lying in ambush there. After Chopper vouched for AP-5, Hera agreed to trust the former Imperial inventory droid. After cross-referencing both Chopper and Imperial data, AP-5 identified the planet Atollon as a safe haven with no Imperial presence.
Using AP-5's information, Hera warned Commander Jun Sato and the other rebels that the Yost system was unsafe. Before AP-5 could transmit the new hyperspace coordinates for Atollon, he and Chopper were accosted by the Imperial ship captain, who had woken up. Following a struggle, Chopper managed to subdue the Imperial but not before the captain shot AP-5 twice with his blaster; severely damaging the droid. Chopper then completed AP-5's work by transmitting the rest of the coordinates to Hera, who in turn relayed them to Commander Sato. Due to AP-5's information, the Phoenix rebels were able to escape the Imperial trap and reached Atollon.
After the rebel fleet reached Atollon, they were rejoined by Chopper and AP-5. After Sabine repaired AP-5, the inventory droid decided to join the rebellion at the advice of Chopper. While the rebel fleet remained in orbit, rebel forces under Captain Rex began building a base on Atollon. In honor of Chopper, the new base was named "Chopper Base." Rebel efforts to establish the base encountered a major obstacle in the form of several predatory spider-like creatures known as krykna. The krykna devoured Lieutenant Dicer and kidnapped Rex, who was subsequently rescued by the Spectres. Unwilling to abandon the base, the rebels constructed a perimeter fence consisting of sensor markers; a security technology that irritated the krykna and kept them away.
The attack on the rebel fleet served as the climax of the Star Wars Rebels episode "The Forgotten Droid," which first premiered on Disney XD on March 16, 2016. It introduced several new characters and plot elements including the inventory droid AP-5 (who was voiced by Stephen Stanton), the Imperial cargo ship captain (James Adomian), and the planet Atollon.