Colonel Sebastian Doyle was the man Lister believed himself to be when attacked by a Despair Squid. While exploring the SSS Esperanto, the Dwarfers were assailed by the Despair Squid, which sprayed Starbug with its hallucinogenic ink. Starbug crashed into a sandbank and the crew were killed upon impact. They then awoke in a Total Immersion Video Game arcade, where they were told that the past four years were actually spent in a simulation.
When they opened the suitcases that contained their "real" identities, Lister discovered that his name was Sebastian Doyle, that he worked for a company called CGI and had some very expensive clothes and jewellery. He even had his own limousine in the car park. He also discovered that Rimmer was really his half-brother, William. Billy Doyle was a smelly, alcoholic drop-out and was his brother's complete opposite.
Later, as they left the arcade to discover more about themselves, the Dwarfers were confronted by a fascist policeman. He threatened to execute them as enemies of the state, until Lister stepped out of the shadows. The policeman recognised him as Voter Colonel Sebastian Doyle - CGI section chief and head of the Ministry of Alteration, an assassination bureau. Rumours had abounded that Doyle had spent the last four years on a personal retreat. Kryten had to kill the policeman to prevent him from killing a fleeing child, so Lister drove them away at breakneck speed from the pursuing authorities.
In spite of his slovenliness, Lister had always considered himself to be a good and honourable man and was devastated by the revelation that he was really the head of an agency responsible for sending thousands of people to their deaths. He was prepared to commit group suicide with the others, until Holly managed to convince Kryten to release a mood stabiliser into the air that brought them all back to reality (RD: Back To Reality).
Although this was only a hallucination, according to dimensional theory this fantasy world continued to exist after the Dwarfers had left, meaning that Doyle's legacy goes on.