Doctor Who: 7 Times The Doctor Was Betrayed By Their Companions

2. Turlough Tries To Kill The Doctor

We're first introduced to the carrot-topped Turlough in the 1983 serial Mawdryn Undead, where, interestingly, he's seen as both a companion, and a villain.

Turlough is brought into the fold after striking a deal with the Black Guardian, a god-like being who offers Turlough the opportunity to leave Earth... in exchange for murdering a so-called "evil creature" known as the Doctor.

Agreeing to this dodgy deal, Turlough proceeds to integrate himself into the TARDIS crew, joining the Fifth Doctor, Tegan, and Nyssa on their travels, pretending to be on their side.

He tries to kill the Doctor several times - whacking him over the head with a rock, sabotaging the TARDIS - but none of his attempts are successful, and eventually, Turlough abandons his murderous quest, and becomes a proper companion for the rest of his tenure.

With that head-bashing attempt in particular though, Turlough came surprisingly close to ending the Doctor's life, and if things had gone his way a bit more, he might've actually succeeded. He wasn't a true companion in his first few serials, sure, but attempted murder is still a betrayal of the highest order.