MacAlester St. John was a Tau'ri who, while working for the CIA as a counterintelligence agent, was involved in an explosion which rendered him disabled. He was then offered the chance to become a Tok'ra host and immediately accepted, joining their ranks.
Born with a slight, but not fragile, build, hair that could not be distinguished between blond and brown, and blessed with a near-photographic memory for both facts and mannerisms, as well as a face that blended into most Western European crowds, MacAlester seemed the perfect choice to become an undercover agent. Perhaps because of these features, MacAlester joined the CIA, becoming a counterintelligence agent, and soon came to view disguise and infiltration as more than merely a career choice but also the most exciting game in the world.
It was during one of his assignments, in Belfast, that a pipe bomb tore through the cab he was in and damaged him severely. MacAlester spent the next two months in an emergency ward where he was forced to cling for his life before eventually being transferred to a secure facility in Bethesda, Maryland. Though his life was saved, it was here that he heard the awful truth; the blast had severed his spinal column which meant that he would never be able to walk again. Because of this, the doctors taking care of him set up a suicide watch as they were convinced MacAlester would try to take his own life due to being unable to perform the job he found so fulfilling.
Before anything drastic could happen, however, MacAlester received a letter from Major General George S. Hammond, a man claiming to have an United States Air Force command within Cheyenne Mountain. Hammond informed MacAlester that he had a group of people he wanted MacAlester to meet. Though MacAlester was sure that nothing could help him, he agreed to meet with these beings. However, when the Tok'ra arrived and explained their offer, MacAlester immediately accepted; he would become the host for the Tok'ra symbiote Antoc and, in return, his injuries would be healed. Shortly thereafter he blended with Antoc, and was informed that his new duties would take him to the stars to face the greatest enemy Earth had ever faced; the Goa'uld. MacAlester accepted his new assignment happily, becoming the second Tau'ri to be a Tok'ra host.
His first assignment from the Tok'ra placed him as a spy within one of Pelops's research laboratories where he collected intelligence on the scientists projects. Before his cover could be blown, MacAlester faked his own death in a rigged explosion and escaped with the information which he returned to the Tok'ra. Soon afterwards, he was sent on his second assignment to the planet Longinus where he disguised himself as a cupbearer—known as Peter Doyle—who had been donated to the Observation Dome by Mordred, one of Manannan mac Lir's underlords. Though this ruse was a lie, he was able to infiltrate the facility. Indeed, MacAlester was secretly amused by the low-class identities he was forced to assume, as he was intimately familiar with society etiquette and enjoyed being able to exploit his ability to read and manipulate people in the subtlest ways possible; from bottom to top.
Since coming to Longinus, MacAlester suspected that the Dome's Majordomo, Kai Yizhi, had some kind of surveillance network set up within the Dome but was unaware as to how completely integrated into the building's structure it was. As such, he made all of his encrypted communications from outside the Dome where they were received by a tiny probe disguised as a piece of space debris which was in geosynchronous orbit above the Dome. This probe then transmitted MacAlester's communications to the Tok'ra intelligence network. In order to evade Yizhi's suspicions, MacAlester made these transmissions irregularly.
MacAlester's ruse was unable to last indefinitely, however, and his real identity was eventually picked up by Yizhi's surveillance network. Yizhi was quick to act, and imprisoned MacAlester and his symbiote within one of the cells which honeycombed the subsurface levels of the dome. Another undercover Tok'ra operative, however, was on the way to Longinus at the same time. The Tok'ra had become intrigued by MacAlester's reports and, as such, sent their operative with an SG team who were tasked with scavenging the Dome for valuable information and get out unscathed while recovering MacAlester so he could be replaced by another Tok'ra operative. Unfortunately for the team, they were unaware that MacAlester had been captured; how the team fared remains unknown, though it is known that Yizhi continued to torture MacAlester for information while he was held captive. (RPG: "First Steps: The Stargate Unexplored Worlds Roleplaying Sourcebook")