Vislor Turlough

When Turlough first appears in the serial Mawdryn Undead, he is a student of retired Brigadier Lethbridge-Stewart (himself a former companion of the Doctor) at the Brendon Public School, but it becomes apparent that he is not what he seems. He is contacted by the malevolent Black Guardian, who offers to take him home if he kills the Doctor. He also appears familiar with concepts of time travel and matter transmission. At the end of the serial, Turlough asks to accompany the Doctor. Despite Tegan and Nyssa's suspicions, the Doctor accepts Turlough as part of the TARDIS crew.

During the course of the next two serials, Terminus and Enlightenment (collectively known, together with Mawdryn Undead, as the Black Guardian Trilogy), Turlough finds himself unable to decide whether or not to carry out his assignment from the Black Guardian, but eventually rejects him in favour of loyalty to the Doctor. Although always slightly cowardly, with excellent instincts of self-preservation and a streak of ruthlessness, his relationship with the Doctor and Tegan improves with time (Nyssa having departed at the end of Terminus). He is one of the few companions capable of operating many of the TARDIS's systems, being able to run a diagnostic in The Five Doctors and program the TARDIS to retrieve the Doctor in Planet of Fire. Initially expressing a desire to return home, he continues travelling with the Doctor and Tegan until Tegan leaves at the end of Resurrection of the Daleks.

In the very next serial, Planet of Fire, it is revealed that Turlough is a junior ensign commander from the planet Trion. Following a civil war on his home planet, in which his mother was killed, Turlough's family were branded political prisoners. His father and younger brother Malkon were exiled to the planet Sarn whilst Turlough himself become a political exile to Earth watched over by an eccentric Trion agent disguised as a solicitor in Chancery Lane. Also revealed for the first time in this serial is Turlough's first name, Vislor. At the end of the serial, Turlough discovers that political prisoners are no longer mistreated on Trion and decides it is time to return home.

An image of Turlough appears during the Fifth Doctor's regeneration scene in The Caves of Androzani.

Strickson has humorously commented that, not knowing what to do with him, the writers of the television series would often have the villains capture or lock him up, leading to Turlough ending up in various "states of bondage".

• Vislor Turlough on Tardis Data Core, an external wiki
• Vislor Turlough on the BBC's Doctor Who website