When he was only six months old, Thonmi was taken to the Monastery of Det-Sen. Songsten saw that he was the reincarnation of the master Thonmi Sambhota, the inventor of the Tibetan script, a minister of the Emperor Songstän Gambo. As per the traditions of Tibet, when they find reincarnations of the great masters who have passed away, they are given a special education to become the teachers of the future. Thonmi was to be the student of Songsten Rinpoche. (PROSE: Night of the Intelligence)
When the Doctor was captured by Khrisong, Thonmi didn't think the Doctor was responsible for the deaths of Edward Travers' companion and four monks. While the Doctor was imprisoned, he gave Thonmi the holy Ghanta that had been missing from the monastery for 300 years. Thonmi brought it to Padmasambhava, who then sent him to Khrisong with orders to set the Doctor free.
Thonmi helped Jamie McCrimmon capture a Yeti. He was then ordered to guard Victoria Waterfield, who sneaked away from him when he went to get food for her. He found her being menaced by the captured Yeti, held the Yeti off so she could escape, and let the Yeti out of the monastery.
When the monks left the monastery for their safety, Thonmi stayed behind to help the Doctor. He taught Victoria the "Jewel of the Lotus" prayer to keep her from succumbing to the Intelligence's hypnotism of her. While the Doctor battled the Intelligence, he and Jamie smashed its equipment, defeating the Intelligence. He sounded the gong to bring the monks back, and said goodbye to the Doctor and his companions. (TV: The Abominable Snowmen)
Thonmi committed himself to nursing Songsten back to health. The more time they spent alone, the more Thonmi learned of Padmasambhava. Eventually, Songsten died, and Thonmi became the abbot of Det-Sen. By that point, he had learned much of Padmasambhava’s techniques in astral projection. On the astral plane, he encountered Padmasambhava and learned his great secret. Over the years Thonmi ministered to Edward Travers and taught him many of the secrets of Det-Sen, including the method of astral projection that would allow him to commune with Padmasambhava. (PROSE: Night of the Intelligence)
In December 1969, Thonmi rescued Travers from the clutches of the Vault and brought him onto the astral plane. A few weeks later, in January 1970, Thonmi met up with Anne Travers and guided her onto the astral plane, after teaching her much about reincarnation and the history of Det-Sen, so she could say goodbye to her father. (PROSE: Night of the Intelligence)