Doctor Who: 7 Times The Doctor Was Betrayed By Their Companions
6. Adric Tells Monarch About The TARDIS
Speaking of less popular companions, Adric is usually near the bottom of the list when it comes to fan rankings, and there are several reasons why this is the case.
Aside from his generally annoying behaviour, a big reason is the fact that he made some questionable decisions over the course of his travels with the Fourth and Fifth Doctors - even going as far as betraying Five in the 1982 serial Four To Doomsday.
This story finds the TARDIS crew - the Doctor, Adric, Tegan, and Nyssa - accidentally landing inside a futuristic ship. From here, they encounter a green alien known as Monarch, who we later learn is intending to destroy the Earth. Villains gonna villain.
It's abundantly clear that Monarch is up to no good, but strangely, Adric slowly begins to warm to his way of thinking, culminating in a scene where he tells Monarch all about the Doctor, Rassilon, and - stupidly - the capabilities of the TARDIS.
The situation only escalates from here, but fortunately, the Doctor soon talks Adric round, and the young companion realises the error of his ways.
While Adric wasn't trying to betray the Doctor in a straightforward sense, this was nonetheless a dumb stunt for him to pull, and things could've got really bad if Monarch had got his grubby mitts on the TARDIS.