Lon was the heir to the Manussan title of Federator when he encountered the Fifth Doctor, Tegan Jovanka and Nyssa. His mother, Tanha, struggled to make him understand the weight of responsibility attached to being effectively a prince, but he was largely indifferent to his destiny. Also, he showed no interest in his people's history, despite repeated attempts by Ambril, a kind of "court historian", to get him to investigate local sacred caves. He stubbornly insisted — erroneously — that even if the Mara had existed on Manussa five centuries prior, it had been destroyed, not merely banished, and was not worth worrying about.
However, when Tegan offered to meet him privately, his ostensible boredom with life temporarily abated. He believed Tegan was making a romantic overture, but as she was under the control of the Mara, she had distinctly different plans. Taking his hand in hers, she brought him under the control of the very being he had refused to take seriously. Lon began to use his power as deputy Federator for the Mara's benefit, which almost led to the Mara's successful return. However, the Doctor confronted and defeated the Mara once again. (TV: Snakedance)