Most of the Doctor's companions are intelligent and respectful folk who understand how lucky they are to be travelling through space and time.
They help the Doctor where they can, stay out of the way when they aren't needed, and, on occasion, even do things that the Doctor isn't willing to do. River's habit of blasting aliens to death certainly came in handy when facing off against the Silence!
But on very rare occasions, we have seen examples of companions behaving badly.
Whether it's a good companion making a series of misguided decisions, or one of those rare bad companions who is acting out of their own self-interests, not all of the Doctor's TARDIS crew-mates have perfect, spotless records - some of them have actually betrayed the Doctor, stabbing the Time Lord right between the shoulder blades.
Sometimes, these betrayals are minor - or even accidental - and are easy to brush aside. But other times, they're huge events that forever change the dynamic between the Doctor and the companion in question, creating some of the most memorable moments from both the classic and modern eras of the show.
7. Adam Mitchell Exploits Knowledge Of The Future
From Susan, Ian, and Barbara, all the way through to Graham, Ryan, and Yaz, most Doctor Who companions - even the less popular ones - have their fans.
But Adam Mitchell, on the other hand? Well... even the most stubborn Whovian probably couldn't bring themselves to side with him.
A short-lived companion to the Ninth Doctor, Adam's temporary TARDIS tenure came to a tense conclusion when he betrayed the Doctor's trust for nothing more than personal gain. Exploring Satellite 5 in the year 200,000, Adam's greed prompted him to try and collect as much future knowledge as he could - mainly technological knowledge - so that he could send it back to his own time.
Basically, just imagine a caveman trying to steal the blueprints for a MacBook.
Upon discovering Adam's meddling, the Doctor, naturally, grew quite angry. As he puts it, just "one second" of that knowledge could change the world for the worse, severely disrupting humanity's natural technological progression.
As a result, the Doctor dropped Adam off in the 21st century, dumping him from TARDIS duties forever. In the show, this is the last we saw of the character, but in the comics, Adam goes on to become a full-fledged villain, seeking vengeance against the Doctor for leaving him behind.