Domino Squad was a military unit of clone trooper cadets stationed on the planet Kamino during the Clone Wars. The members of the squad, at first, proved unable to effectively work together as a team, but they ultimately passed their tests on Kamino and became full members of the Grand Army of the Republic.
After graduation, they were stationed at Rishi Station on the Rishi Moon, a strategic point on a route to Kamino. The majority of Domino Squad perished: CT-00-2010 "Droidbait" died during a Separatist attack on the base, CT-4040 "Cutup" was eaten by a Rishi eel, and CT-782 "Hevy" sacrificed himself to blow up the outpost. The survivors CT-1409 "Echo" and CT-5555 "Fives" were subsequently absorbed into the 501st Legion. During a mission on Lola Sayu, while escaping, Echo noticed a commando droid targeting a shuttle with a laser turret. He advanced to secure their ship, ultimately being blown up with the ship in a massive explosion that led his comrades to believe he was killed. Despite the loss of Echo and Piell's death later during the mission, it was ultimately a success.
While on a mission in Ringo Vinda, Fives saw CT-5385 “Tup” killing Jedi Master Tiplar and finding out all the clones had inhibitor chips in their heads which made them follow orders without question, Fives was accidentally killed by CC-1010 "Fox" before he could reveal Palpatine's plan, which led to Order 66.
Unbeknownst to everyone, Echo survived the explosion, but was gravely injured and was taken prisoner by Separatist forces and kept alive via cybernetics. Eventually CC-7567 "Rex" and Clone Force 99 rescued him.
Echo was the last surviving member of Domino Squad.
Domino Squad were initially full of conflict.
Members of Domino Squad were clones of the bounty hunter Jango Fett and born on Kamino. During the Clone Wars, the members of Domino Squad started their last practice tests in the Citadel Challenge under Master Chief Bric and the bounty hunter El-Les. During their tests, the squad was not able to work together as a team, their lack of cohesion causing numerous failures. During their final test, the squad failed when they left CT-00-2010 "Droidbait" and were chastised by the ARC trooper Clone Commander Colt for their actions. Despite the setback, El-Les convinced Jedi General Shaak Ti to let the squad retake the final test. CT-782 tried to go AWOL, but was convinced to stay by 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.
Domino Squad saw their first action on the Rishi Moon.
The members of the squad were assigned to the Rishi Moon's outpost under the command of Sergeant O'Niner. Following a meteor shower, the listening post was taken over by a group of commando droids that had arrived to the surface in droch-class boarding ships believed to be meteorites. Droidbait was killed in the process, while the rest of the members of Domino Squad, CT-5555 "Fives," CT-1409 "Echo," CT-4040 "Cutup," and Hevy, escaped onto 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 only remaining squad members, and Rex and Cody were picked up by the Republic after Grievous' fleet fell back. 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.
Fives became aware of a conspiracy regarding the inhibitor chips placed inside each clone.
Whilst fighting alongside Tup in the Battle of Ringo Vinda on the planet of Ringo Vinda, Fives witnessed Tup suddenly murder Jedi General Tiplar in the midst of heated combat. In the course of an investigation into Tup's sudden illness, Fives discovered the existence of the inhibitor chips designed to keep the 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. Believing that the chips were part of a sinister conspiracy, Fives was sent to Coruscant to talk to Chancellor Sheev Palpatine, only to discover that Palpatine was not only aware of it but the mastermind of the plan: Order 66. Palpatine framed Fives for trying to kill him and set the Coruscant Guard on his tail. Fives escaped and contacted fellow 501st trooper CT-6116 "Kix," whom he tells to contact Anakin and Rex and to meet him in the Warehouse district. Once they had arrived, Fives attempted to convince them of the truth behind the conspiracy and the Chancellor's involvement in it. Before he could make any genuine progress towards this, 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." As Fives was believed to be the sole survivor, Domino Squad came to an apparent end with his death.
Echo joined Clone Force 99 in the aftermath of Anaxes.
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 was rendered a cyborg after being experimented on and altered by Emir Wat Tambor and the Techno Union, who hoped to use his memories of Republic strategies, specifically Rex's battle strategy algorithm, to help the Separatists win the Battle of Anaxes. After being rescued from Skako Minor by Skywalker, Rex, and the "Bad Batch," Echo was vital to the victory of the Republic forces on Anaxes, and was named Clone Corporal for his heroics. Now that he was a cyborg, and feeling at home with the irregular clones, Echo decided to join the Bad Batch and left Anaxes with them. Echo continued to accompany Clone Force 99 until the end of the war, where they witnessed the execution of Order 66 and the transition from the Galactic Republic to the Galactic Empire. They fled the new regime and went on the run.