"Relics of the Old Republic" marks the fourth installment in the Star Wars Rebels animated television series' second season. It is, in total, the nineteenth episode presented in the series.
Following its initial showing at New York Comic Con on October 8, 2015, the episode was broadcast on October 21, 2015, via the Disney XD channel.
The Spectres' location on the planet Seelos has been discovered by the Galactic Empire thanks to the actions of Wolffe. Admiral Kassius Konstantine and Agent Kallus of the Imperial Security Bureau journey to the Seelos system with the intent of either capturing or eliminating the rebel group. Sabine Wren is repairing the Phantom inside the clones' modified All Terrain Tactical Enforcer when Kanan Jarrus contacts her to inquire about her progress. He then reaches out to Hera Syndulla, who is in orbit aboard the Ghost, working on the ship's hyperdrive. She informs him that there are no signs of the Empire, just before the Imperial Star Destroyer Relentless emerges from hyperspace, prompting Hera to make a frustrated remark about tempting fate. In haste, she powers down all of the starship's systems, including her astromech droid Chopper, who voices his displeasure, to avoid detection by the Empire.
Before they separate, Rex hands Ezra Bridger a datachip that contains a list of former Republic bases located in the Outer Rim Territories and Mandalore, along with some old Separatist bases. Ezra suggests that the clones should join the rebellion, but Rex respectfully declines, stating that he has witnessed too much conflict. Nevertheless, he, Gregor, and Wolffe volunteer to stay behind to delay the Empire, allowing the Spectres to escape on the Phantom and rejoin the Ghost in space. Agent Kallus contacts the clones, demanding that they surrender the rebels. Wolffe denies the presence of any Jedi or rebels, attributing it to a malfunction in his cybernetic eye. Kallus reveals that he possesses images from the probe droid that confirm the rebels' presence, at which point Rex joins the conversation and informs the ISB agent that if he seeks a fight, he should have brought soldiers of higher caliber than Imperial stormtroopers. Kallus responds smugly that they are loyal and numerous, to which Rex retorts that he will need all of them before ending the transmission.
A sandstorm arises near the modified AT-TE, preventing the rebels from leaving. Agent Kallus dispatches three AT-AT walkers to destroy the clones and rebels. Rex proposes that they flee into the sandstorm, with Kanan and Ezra Bridger using their Force abilities to guide the walker. This would blind the Imperials, but not the rebels. Despite his distrust of the clones, Kanan agrees to the plan. He guides the AT-TE into a position where they are surrounded by Kallus' three AT-ATs, telling Rex that this is their best chance to destroy one of them. Ezra and Sabine are sent to the AT-TE's main gun to make the shot, with Rex lending the Padawan his helmet for protection from the storm. Guided by Kanan, Ezra fires, striking one of the AT-ATs in the neck and destroying it, enabling the clones' walker to escape the remaining two.
In space, Admiral Konstantine receives a transmission from Darth Vader, ordering him to rendezvous with his shuttle in space. Konstantine objects, arguing that this will compromise Kallus' ground mission, but the orders are clear, and the Relentless leaves the system. Hera, working with Chopper, whom she had reactivated at low power, observes this. Unsure of what the Star Destroyer's departure signifies, she instructs Chopper to continue repairing the ship.
After emerging from the sandstorm, the Spectres prepare to depart on the Phantom to rejoin the Ghost in space. Despite Ezra's desire to stay and fight, Rex orders Ezra to leave, as the clones prepare to defend against Kallus and his two remaining AT-AT walkers. As the rebels leave on the Phantom, Kallus' AT-AT walker manages to overpower the clones' walker. Before Kallus can destroy the clones, Ezra persuades Kanan and the other rebels to return and assist the clones, emphasizing that the old soldiers are willing to fight to the death.
Although Kanan still has reservations about the clones, he agrees with Ezra and pilots the Phantom back into the battle. While Sabine flies the shuttle, Kanan, Ezra, and Garazeb Orrelios seize control of one of the AT-ATs and turn its weapons on Kallus' AT-AT walker. With his AT-TE no longer pinned under the AT-AT walker's legs, Rex is able to destroy Kallus' walker with a well-placed shot. Kallus and his remaining troops flee the battle on a speeder, while the clones salute Ezra and Kanan. Rex agrees to join the rebels and accompanies them back to Phoenix Cell.
In the meantime, Admiral Konstantine welcomes the shuttle that landed in his Star Destroyer's hangar. Instead of Vader, the Admiral is surprised to find the Fifth Brother, a Force-sensitive Inquisitor. When Konstantine mentions that his diversion may have jeopardized Kallus' mission, the Fifth Brother expresses indifference and asserts that he will succeed where Kallus and Konstantine have failed.
Upon returning to the Phoenix rebel fleet, the Spectres and Rex are reunited with Ahsoka Tano. When Rex remarks that Ahsoka has aged, the former Jedi Padawan replies, "It had to happen sometime, Rex" as they embrace. The Spectres, Ahsoka, and Rex then walk off together.
In Rebels Recon #2.06: Inside "Wings of the Master," Lucasfilm Story Group representative Pablo Hidalgo verified that Gregor and Wollfe remained on Seelos and made the remaining AT-AT walker their new residence.