After acting in a malevolent and mocking way in the previous episode (the highlight being his attempt to electrocute Ian) the Doctor’s in a much more friendly and mellow mood in The Cave of Skulls. Was this inconsistent scripting or intentional – ensuring that the audience would be forced to keep guessing about his ultimate motivations?

But maybe he only mildly rebukes Ian’s continuing inability to accept the situation (“You really are a stubborn young man, aren’t you?”) because he’s now in command, having left London in 1963. Whatever the reason, the Doctor’s keen to explore (which will, for the first but not the last time, get him into trouble).

If the title of 100,000 BC is to be believed, then the Doctor’s correct in his assumption that they’ve travelled back in time. It’s interesting though that this is never confirmed on screen – it’s explicitly stated that the TARDIS’ “yearometer” isn’t functioning and so the date can’t be confirmed.

Had it been revealed at the end of episode four that this was actually a future vision of the Earth, following a nuclear holocaust, then it wouldn’t have come as a surprise. Maybe this was the original intention but got lost after one of the many rewrites? Not that it really matters, but it would have given the story an extra little frisson.

The tribe are a mixed bunch. Most mannered is Howard Lang as Horg who’s difficult to take seriously, although Derek Newark (Za), Alethea Charlton (Hur), Kal (Jeremy Young) and Eileen Way (Old Mother) are much better. All would return to the series in later years, as well as popping up in numerous other series of the time.

Za, Hur and Kal form an unlikely love triangle with Old Mother looking on ironically from the sidelines, constantly muttering that it would be better if Za never learns the secret of fire. It’s hard to understand her vehemence against fire, especially if one believes Za’s statement that without fire they’d die. Exactly why fire strikes such fear into her heart is never explained.

The initial TARDIS scene is notable for allowing the doors to open on the alien landscape. This wasn’t very common (although it would crop up again in The Sensorites) but I’m grateful they did it here since it really helps to sell the illusion of stepping from the ship into the unknown.

The forced perspective sets of the apparently endless plains may be obvious if you look too hard, but given the small amount of money Barry Newbery had to play with they’re still impressive. The wind sound effects help to create the impression that it’s freezing (although that makes the moment when Ian touches the sand and is astonished how cold it is, all the more strange).

We get the first of Susan’s hysterical fits, when the Doctor disappears – I really wish Barbara had slapped her hard as it might have discouraged her from doing it again! As for the Doctor, we see him enjoying a crafty smoke with a pipe – clearly this was only introduced so that Kal could see the Doctor make fire (or maybe the trauma in the Cave of Skulls was the moment he decided to kick the habit?)

Although Ian’s still in denial about everything, there’s also the first sign of his practical nature – after the Doctor disappears he automatically takes command. And when they’re all imprisoned in the Cave of Skulls it’s no surprise that Barbara is the one he checks on first (“Are you all right? Did they hurt you?”). Although never explicitly stated on-screen it seems obvious Barbara and Ian are very much a couple (as David Whitaker later confirmed in The Crusaders novelisation).

With the Doctor having seemingly lost all of his previous bluster (“Oh, I’m sorry. I’m sorry, it’s all my fault. I’m desperately sorry.”) things look bleak for our four heroes as they contemplate the myriad of skulls – all of which have been split open.