Doctor Who: 7 Times The Doctor Was Betrayed By Their Companions
4. Dodo Attempts To Brainwash The Doctor
First Doctor companion Dodo Chaplet got a rather unceremonious sendoff in the 1966 serial The War Machines, with the character being written out offscreen.
The in-universe reason for this was that Dodo had been taken to a remote country house to recover from her encounter with WOTAN, a sinister machine that brainwashed her and tried to get her to recruit the Doctor for its villainous scheme.
WOTAN is so advanced that it manages to hypnotise Dodo within minutes of meeting her, and her ordinary human mind is powerless to resist. The machine then tells her that "Doctor Who is required", and sends her off to bring him in - a task that she comes dangerously close to completing before her mind is freed from WOTAN's grip.
Since she was acting against her will, Dodo obviously didn't mean to work against the Doctor here, but her actions were a betrayal nonetheless.
And funnily enough, Dodo isn't the only companion who's been brainwashed into betraying the Doctor. This is something that happens quite a lot throughout the show, from Sarah Jane Smith in The Hand Of Fear, to River Song in Let's Kill Hitler.
The Doctor is a tough nut to crack, so often, it's best to try and get to them through their more vulnerable companions!