Sean Dunston was born in 2243 on Avalon, the son of a Tech Infantry soldier and a nurse. His father died fighting the Vin Shriak in 2252, his mother five years later when her hospital ship was destroyed by a Vulthra convoy raid in hyperspace. Spending much of his teenage years in Federation-funded boarding schools, he knew from a young age that his destiny was to join and serve in the Tech Infantry. He voluntarily accepted being drafted into the TI at age 16 and was soon figthing in the Arachnid Quarantine Zone.
After two years fighting the Bugs, he was reassigned to a Marine detachment on the EFS Poseidon, the Star Control Ship which was the flagship of Admiral Joseph Smythe. When the Caal Invasion came in late 2264, Smythe refused to either bring his fleet to the defense of Avalon or to accept the dissolution of the Federation by Vin Dane and the founding of the Holy Terran Empire. Private Dunston was one of his TI Marines sent to bolster the defenses on the southern frontier once it became clear that the Poseidon would be stuck defending the new Federation capital on New Madrid for the foreseeable future and thus unable to participate in offensive actions and unlikely to have great need for its embarked TI Marine Legions. When Kalintos was invaded by the Imperial military, Dunston was attached to a newly-formed unit consisting of ordinary TI soldiers and Raptor special forces to expel the Imperial forces from Kalintos. During the space battle that opened that campaign, he was wounded during the capture of the INS Alastar Dimiye. He recovered quickly and participated in the ground fighting during the liberation of Kalintos, including the mission that killed Lord General Nicolai Malakov.
Afterwards, he and his unit were sent to invade and liberate St. Michael's Star. While en route to this next operation, Dunston saved the life of his squad leader, Argus McCall, when an assassin tried to kill him during a training session, apparently in revenge for McCall exposing a conspiracy of werecreatures anxious about the loss of the favored status they enjoyed under Arthur Clarke in a new Federation run by Joseph Smythe.
During the liberation of St. Michael's Star, Dunston and his fellow troopers were stranded on the surface when the Earth Fleet task force was forced to withdraw. He took part in the vicious fighting in and around Port Prosperity, this time protecting his commanding officer, General Nasrudin Carson, rather than seeking out the enemy commander for assassination. As the Federation forces on the planet were slowly whittled down and squeezed into just the capital city, the troops were rescued at the last minute by the timely arrival of Dunston's old ship, the EFS Posdeidon, which defeated the Imperial Fleet task force and extracted the survivors of the invasion force. Following the bombardment from orbit and utter destruction of Port Prosperity, the Imperial general finally surrendered the system to the Federation.
Dunston and the rest of Soti's Slammers were next sent to Chalfont to capture a Bug Queen and find out why the Bugs had been acting strangely lately. While on this mission, Dunston and the rest of Sixth Squad were ambushed in an underground tunnel by a trio of Warrior Bugs, and Dunston was shot and killed.