The most quoted bit of The Forest of Fear comes in the first couple of minutes: ‘Fear makes companions of all of us.’ It’s a charming moment, beautifully played by Hartnell and Hill. But it’s atypical of the episode, which elsewhere pits the Doctor’s self-centredness against Barbara’s compassion. This is most represented in the moment when the Doctor picks up a rock, clearly to finish off the injured Za and make good his and Susan’s escape (Ian’s visible disgust at this point is one of the best William Russell moments). It’s the third time in the episode that a lot of focus is placed on a hand clutching a sharp rock. In the opening sequence, Old Mother steals Za’s knife, and Waris Hussein dwells on it, in close up, for several seconds. Later, there’s a focus on Kal’s knife as he finishes off Old Mother. These are all the moments of most apparent peril.

But the episode hinges less on these than on two moments of compassion. The first is Old Mother’s. Creeping into the Cave of Skulls, she frees the time travellers on the proviso that they do not make fire. Then, later Barbara impulsively sacrifices her freedom to help Za and Hur. Though the Season One production team would never have explicitly allowed it, Barbara may well be inventing humanity here, introducing concepts like friendship and cooperation to the Tribe of Gum, who, it’s implied, are among the few surviving human beings at this time.

It’s really interesting that while the chest-beating focus is on the leadership of the tribe, between Kal and Za, and a similar power struggle between the Doctor and Ian for leadership of the TARDIS crew, it’s Barbara and Hur who really lead. Hur cleverly manipulates Za into action, while Ian, Susan and ultimately the Doctor reluctantly follow Barbara’s lead at the crucial moment of Za’s injury. While it’s a stretch to claim a feminist message for the episode, it’s interesting to see such strong female characters in a series that actually gets worse at this in the later 1960s. It also makes up for Barbara’s rare (but, in context, understandable) hysterics earlier on in the episode.

Elsewhere we learn a bit more about the Doctor – he’s not a doctor of medicine, for a start. At the beginning of the episode he’s behaving like an aloof observer, critiquing Ian’s efforts. To his credit, when Ian rightly calls him out on this, he rises to the bait and uses his intelligence to propose a practical solution to their predicament. Elsewhere, he’s proud, but also smart enough to accept Ian’s suggestions for the good of the group.

Although the cavemen episodes have been a bit derided over the years, this is extremely well directed – the beast attack on Za, without any expensive effects (or, indeed, beasts), absolutely sells what’s happening. And the cliffhanger, with the tribe rising from rocks and dunes around the TARDIS, is chilling.

Next episode: The Firemaker