There have been many companions in Doctor Who over the years, but very rarely, if ever, have they ever had a physical argument that wound up with one of them being unconscious. But this is exactly what happens in Episode Three when Tegan throws Adric aside after the two have a blazing row.
Adric's decision to side with Monarch is the major catalyst of this argument starting, but blame must also be put on Tegan, who is in such a frenzy over the prospect of Monarch's invasion of Earth that she's as irrational as any companion seen in the series. Before her argument with Adric, Tegan had an equally volatile spat with The Doctor, who, despite the centuries of experience in situations involving megalomaniacs, is unable to convince his bossy Australian friend that he might be the more qualified of the two to handle their current dilemma.
To have one companion so shockingly at odds with the rest of her crew mates is an inevitability of having three companions in the TARDIS. In order to create some sort of drama for all three of them, one companion has to become at odds with the rest of the team, because it is too difficult to create tension in three or four separate storylines. In this story, Tegan is more or less left on her own, either locked up in her quarters, or stuck in the TARDIS foolishly trying to pilot it back to Earth. In other scenes, she merely tags along with The Doctor, offering little apart from a staggering ability to speak the same language of a 20,000-year-old Australian Aborigine, and showing equally great skill as a sketch artist.
Look at how The Doctor reacts to Adric telling him that Tegan has taken the TARDIS for a joyride - he's not worried or angry, he's annoyed, like a parent to a 15-year-old who's just taken the keys to the family Buick. This is another inevitability of the current Doctor/companions matchup, especially when The Doctor looks so young. If a Doctor who looks to be in his late 20s is the authority figure, then how young do the companions have to be in order to create a believable visual age gap between Doctor and companions? Answer: teenagers, or at least the behavioural capabilities of teenagers. Four To Doomsday is a delightful story, but at its heart is a production team that doesn't yet know how to deal with the youngest looking Doctor in the programme's long history.