Sub-Technician Thon was a Threllip sent to Earth in 1982 with Captain Vengorr. He worked with Vengorr for 32 years. Despite his lowly position, Thon was intelligent and philosophical. He resented Vengorr, but generally did his job without complaints.
In 1982, Vengorr and Thon were sent to Ferrara, Italy to set up a transdimensional portal so that the Threllips could invade. The Eighth Doctor and Charley stopped them by pretending to be Time Lords, sent to catalogue the turning point in history for the Threllip Empire. They claimed that Vengorr would become the leader of the new empire and Earth was to be the first planet he would conquer. While the Doctor was showing Ferrara to Vengorr, Thon stayed behind to set up the transdimensional portal. When Thon asked about his fate in history, Charley claimed he would be forgotten completely except as the name of Vengorr's pet wartsnoggler. This added to Thon's considerable resentment and when Vengorr returned, he attacked him. While they were fighting, the Doctor was able to reprogram the transdimensional portal, sending Vengorr and Thon to one of the moons of Mordelius Prime. (AUDIO: Living Legend)