Domino Squad was a military unit of clone trooper cadets based on the planet of Kamino during the period known as the Clone Wars. Initially, the members of this squad struggled to function as an effective team. However, they eventually overcame these challenges, successfully completing their training on Kamino and earning their place as full-fledged members of the Grand Army of the Republic.
Upon their graduation, they received their assignment at Rishi Station, situated on the Rishi Moon, a strategically important location along a route leading to Kamino. Most of Domino Squad met their perished there: CT-00-2010, known as "Droidbait," was killed during a Separatist attack on the base. CT-4040, nicknamed "Cutup," became prey to a Rishi eel, and CT-782, or "Hevy," made the ultimate sacrifice to detonate the outpost. The surviving members, CT-1409 "Echo" and CT-5555 "Fives," were subsequently integrated into the ranks of the 501st Legion. During a mission on Lola Sayu, while attempting to escape, Echo noticed a commando droid targeting their shuttle with a laser turret. He moved forward to secure their ship, ultimately being caught in a massive explosion with the ship, leading his comrades to believe he had died. Despite the loss of Echo and the later death of Piell during the mission, the mission was ultimately completed.
During a mission on Ringo Vinda, Fives witnessed CT-5385 “Tup” assassinating Jedi Master Tiplar. He then discovered that all clones possessed inhibitor chips in their brains, compelling them to obey orders without question. Before Fives could expose Palpatine's scheme, which would lead to Order 66, he was accidentally killed by CC-1010 "Fox."
Unknown to all, Echo had survived the explosion, although he was severely wounded. He was captured by Separatist forces and kept alive through the use of cybernetic enhancements. He was eventually rescued by CC-7567 "Rex" and Clone Force 99.
Echo ultimately became the last surviving member of Domino Squad.
The members of Domino Squad originated as clones derived from the bounty hunter Jango Fett, with their genesis occurring on Kamino. As the Clone Wars unfolded, Domino Squad embarked on their concluding practice assessments within the Citadel Challenge, overseen by Master Chief Bric and the bounty hunter El-Les. Throughout these evaluations, the squad exhibited an inability to function cohesively as a unit, resulting in repeated failures stemming from their lack of teamwork. During their culminating test, the squad faltered when they abandoned CT-00-2010 "Droidbait," prompting a reprimand from the ARC trooper Clone Commander Colt for their actions. Despite this setback, El-Les persuaded Jedi General Shaak Ti to grant the squad a chance to retake the final test. CT-782 contemplated desertion, but was convinced to stay by the malformed maintenance clone 99. Despite Bric's attempt to sabotage the test by stealing their ascension cables, the squad worked together and won the Citadel Challenge. The squad graduated, and amidst their celebrations, CT-782, whom 99 had nicknamed "Hevy," gifted the old clone his medal, saying that he deserved it and he could get it back from him at another time. The squad shipped out to fight the Confederacy of Independent Systems Droid Army.
The members of the squad received orders to report to the Rishi Moon's outpost, operating under the command of Sergeant O'Niner. In the aftermath of a meteor shower, the listening post was seized by a contingent of commando droids that had arrived on the surface aboard droch-class boarding ships disguised as meteorites. Droidbait lost his life during this event, while the remaining members of Domino Squad, CT-5555 "Fives," CT-1409 "Echo," CT-4040 "Cutup," and Hevy, managed to escape to the surface. However, Cutup was soon killed and eaten by a Rishi eel, and the remaining trio warned Clone Commander Cody and Clone Captain Rex about the commando droids by using a droid attack flare. The squad met up with the two officers and retook the base from the commando droids. However, the droids had hot-wired the all-clear signal, and General Grievous dispatched reinforcements to secure the base. The squad and the two officers decided to destroy the base in order to warn the Republic about the planned attack on Kamino. To this end, the group used liquid tibanna as an explosive. The officers and squad fought the droids before abandoning the base. But the detonator would not link, so Hevy stayed behind and sacrificed himself by detonating the gas to destroy the base to warn the Republic.
Echo and Fives, now the sole surviving members of the squad, along with Rex and Cody, were rescued by the Republic after Grievous' fleet retreated. Echo and Fives were decorated by Jedi Generals Obi-Wan Kenobi and Anakin Skywalker and joined Rex's 501st Legion. During their service, the pair eventually fell back to defend Kamino from Separatist attack. The group was reunited with 99, and had to pass on the unfortunate news of Hevy's death, in turn learning about Hevy's graduation medal. Later, Echo, Fives, 99, Cody, and Rex helped a group of clone cadets aid in the battle against a large group of battle droids, but as they exchanged fire in the barracks, 99 was shot and killed. The clone who had been an inspiration to the entire squad died in Echo's arms defending his homeworld.
During the second year of the Clone Wars, Echo and Fives were sent on a mission to Lola Sayu with Generals Kenobi and Skywalker, along with Rex and Cody, to rescue Master Even Piell, who had been captured by Separatist forces. As they were commencing their escape from the planet with Master Piell, Echo was presumed killed while trying to board their ship when it was destroyed, leaving Fives as the apparent sole survivor of the Domino Squad.
While engaged in combat alongside Tup during the Battle of Ringo Vinda on the planet Ringo Vinda, Fives observed Tup suddenly assassinate Jedi General Tiplar amidst the chaos of battle. During an investigation into Tup's sudden illness, Fives discovered the existence of inhibitor chips, which were designed to keep clones docile, but also intended to make them execute an unknown action. Medical droid AZI-345211896246498721347 removed Tup's chip in an attempt to save him, but Tup was too weakened from both the surgery and the degradation of his chip and died. Suspecting that these chips were part of a nefarious plot, Fives journeyed to Coruscant to speak with Chancellor Sheev Palpatine, only to uncover that Palpatine was not only aware of the plot but was also its mastermind: Order 66. Palpatine framed Fives for attempting to assassinate him, unleashing the Coruscant Guard to pursue him. Fives managed to escape and contacted fellow 501st trooper CT-6116 "Kix," instructing him to alert Anakin and Rex and to meet him in the Warehouse district. Upon their arrival, Fives attempted to persuade them of the truth behind the conspiracy and the Chancellor's involvement. However, before he could make any headway, Fives was confronted by troopers of the Coruscant Guard and drew a blaster on them in a panic, only to be fatally shot by CC-1010 "Fox." With Fives believed to be the last survivor, Domino Squad seemingly met its end with his death.
However, during the Outer Rim Sieges, in the third and final year of the Clone Wars, Echo was discovered to be alive on the planet Skako Minor by Captain Rex, General Skywalker, and Clone Force 99, a group whose name honored 99. Echo had become a cyborg after undergoing experimentation and alterations at the hands of Emir Wat Tambor and the Techno Union, who sought to exploit his memories of Republic strategies, particularly Rex's battle strategy algorithm, to aid the Separatists in achieving victory during the Battle of Anaxes. Following his rescue from Skako Minor by Skywalker, Rex, and the "Bad Batch," Echo played a crucial role in securing the Republic's victory on Anaxes, earning the rank of Clone Corporal for his heroic actions. Now a cyborg, and feeling a sense of belonging among the unconventional clones, Echo chose to join the Bad Batch and departed Anaxes with them. Echo continued to accompany Clone Force 99 until the war's conclusion, where they witnessed the execution of Order 66 and the transformation of the Galactic Republic into the Galactic Empire. They fled the new regime and went on the run.