Antoc is a Tok'ra symbiote that is mixed with MacAlester St. John. But when Tok'ra came to Stargate Command and explained their offer, MacAlester immediately accepted, he would be hosting the Tok'ra symbiote Antoc and in return, his injuries are healed. Shortly thereafter he blend with Antoc, and was informed that his new duties would take him to the stars in order to meet the main enemy Earth ever faced, the Goa'uld. MacAlester accepted his new assignment happy, became the second Tau'ri to be a Tok'ra host. His and MacAlesters first assignment was to a spy in one of the Pelops's research laboratories where he gathered intelligence on the researcher projects.
After his host had faked his own death in a rigged explosion and fled with the information that he returned to the Tok'ra. Soon thereafter, he was sent on his second mission on the planet Longinus where he disguised himself as a cupbearer known as Peter Doyle that had been donated to the observed dome of Mordred, a Manannan mac Lirs underlord. While this trick was a lie, he could infiltrate the facility. In fact MacAlesters secretly amused by the low class identities he had to assume that he was very familiar with the community label and enjoyed being able to use their ability to read and manipulate people in the most subtle way possible, from bottom to top.
Since joining the Longinus MacAlester suspect that the dome Majordomo Kai Yizhi, Yizhi, had some form of monitoring network established within the dome but was aware of how fully integrated into the building structure it was. As such, he made all his encrypted messages from outside the dome, where they were received by a small probe disguised as a piece of space debris that was in geosynchronous orbit above the dome. This probe is then sent MacAlester communications to Tok'ra intelligence network. To avoid Yizhi suspicions did Macalester these broadcasts irregularly.
MacAlesters list was unable to indefinitely, however, and his true identity finally by Yizhis monitoring network. Yizhi was quick to act, and imprisoned MacAlester and his symbiote within one of the cells that cavernous subsurface levels of the dome. Another undercover Tok'ra operative, however, was about to Longinus simultaneously. Tok'ra had become fascinated by Macalester reports and as such sent his operative with a SG team who was instructed to dispose of the dome for the valuable information and get out unscathed while recovering MacAlester so that he could be replaced by another Tok'ra operative. Unfortunately for the team, they were unaware that MacAlester had been caught, how the team fared is unknown, although it is known that Yizhi continued to torture MacAlester information while he was held captive. (RPG: "First Steps: The Stargate Unexplored Worlds Roleplaying Sourcebook")