An assassin was tasked by the Grand Teks of the Tharsis Acumen with assassinating the Sixth Doctor in revenge for his foiling their experiments on political prisoners. While the Doctor and Evelyn Smythe flew kites provided to them by the residents of a planet home to a thick mass of air which made visibility somewhat difficult, the assassin shot the Doctor in the neck with a microscopic dart containing a conglomeration of Texineurons. The Texineurons began to ravage the Doctor's body, gradually dragging him into agony and ensuring he would die in 100 days.
With the help of Evelyn Smythe, the Doctor searched his timeline for an antidote, finding it in the assassin's backpack just after he had been shot. He tricked the assassin into believing he was inhaling a container of the virus, prompting the assassin to retrieve the antidote from his backpack as the Doctor and Evelyn walked away. The Doctor grabbed the antidote and injected himself with it, eradicating the Texineurons in his system. He advised the baffled assassin that he would only need worry about to explaining his failure to the Tharsis Acumen. (AUDIO: The 100 Days of the Doctor)