Unlike some other sophisticated gen-three synths, D1-T8 was never made with the intention of sending him to the surface. He instead worked in the Institute's bio-science division from the first week after his creation, where he was responsible for furthering its understanding of bio-engineering, including the effects of experimental pharmaceuticals on organic matter. Under the leadership of Dr's Syverson and Brian Vergil, D1-T8 helped administer a study into the introduction of the Forced Evolutionary Virus (FEV) to human physiology. Though willingness of the test subjects was debatable at best, D1-T8 appeared only mildly concerned about the ethics of such a project; official reports from the time note his sympathetic demeanour, "alongside a conclusive understanding of the value of the Institute's legacy of scientific innovation".
After the death of Dr. Syverson, Virgil took the opportunity to sabotage the project, destroying the majority of the research before fleeing the Institute. Though he was absolved of any culpability in "Incident V", D1-T8 was swiftly reassigned to the bio-agricultural crop research effort. However, Virgil's actions proved integral in causing D1-T8 to question the morality of the Institute's actions. He began communicating in secret with other doubters and, through them, learned of the existence of an organization helping escaped Institute synths in the Commonwealth.
D1-T8 eventually escaped the Institute the help of the Railroad, buying his passage with classified information about the FEV experiments. Whilst the Railroad initially moved to smuggle him out of Boston immediately, D1-T8's first experience of the surface convinced him that he wished to remain in the Commonwealth in order to atone for the damage done by years of unethical science, heedless of the danger of such a decision. Under the assumed name "Delta" he relocated to the town of Goodneighbor and reinvented himself as a trustworthy neighbourhood doctor, treating patients using the medical expertise he had picked up whilst experimenting on their friends and family.
Over the years, Delta cultivated a reputation as an experienced and reliable caregiver - not just amongst the people of Goodneighbour, but also the neighbouring Diamond City, and beyond. Despite his faith that synths and humans could one day peacefully coexist, Delta's survival relied on concealing his true nature from his new peers. His ongoing relationship with the Railroad meant that his clinic soon became a secret halfway house for synths escaping persecution. During this time Delta continued to provide medical care out of the same building, risking discovery by the people of Goodneighbour, or worse.
Sometime around 2287 Delta played host to ex-Brotherhood Paladin Danse who, through the Sole Survivor's interference in the Institute, was revealed to be a synth sleeper agent. His involvement soon bought him to the attention of the Brotherhood and, in turn, the Institute, whose attempts to find and access Delta had thus far been successfully stymied by the Railroad. The Institute promptly dispatched a team of Courser synths to Delta's clinic in Goodneighbour.
Delta and Danse were forced to flee Boston in the dead of night, grudgingly abandoning the town to the Courser attack. They were assisted in their escape by a Railroad operator with the codename of "Wanderer". It was Wanderer who suggested that they might be able to find sanctuary in the Capital wasteland - provided, of course, that they could make the perilous journey through New York. The pair set out for the town of Rivet City, aided as much as possible by the Railroad's increasingly threadbare network of agents, informants, and refugees.
Despite the Sole Survivor's interference, the Institute are still determined to stop Delta before he can reach the safety of Washington D.C. With a number of Coursers hot on their heels, Delta's very presence now threatens the safety of all those kind enough to take him in. It's a hard pill for the good doctor to swallow.
A note on Ships: Delta/Danse began with Delta introducing Danse to a new way of living outside the Brotherhood, and ends with Danse protecting Delta on their frantic flight from Massachusetts. A more cynical person would say that Danse has simply taken his loyalty to the Brotherhood and put it all into Delta instead, but I actually really just like the idea that these two characters from very different backgrounds end up teaching each other.