In the first episode, the Doctor talks about the Red Indian seeing the first steam train. The Cave of Skulls begins with a caveman seeing a time machine materialising from thin air. Inside the TARDIS the time travellers are still arguing. Ian still refuses to believe – clinging to his old ideas. Barbara is more imaginative. I love the shot of the TARDIS doors opening to reveal the frozen sands of prehistoric Earth. Outside, the Doctor is perturbed by the TARDIS’ failure to change shape. It’s the first hint we get that he doesn’t really have any idea how to properly work the Ship. Susan remains odd – prone to hysterical outbursts when she realises the Doctor has been kidnapped by the watching caveman.

The second act focuses on the Tribe of Gum. They are deeply divided, not just by Kal and Za’s rivalry for Hur and the leadership, but by the rift between Old Mother, who believes ‘It is better we have lived as we have always done’, and Za, whose power depends on his ability to make fire. This sets up one of the themes of the first season: the conflict between those who cling to tradition, and those trying to change the world – we’ll see it again in the Thal tribe, in Aztec Mexico, and revolutionary France. The tribe’s way of life is about to be disrupted by the arrival of the mysterious Doctor, Kal’s firemaker. RTD used to make great play with this, having the Doctor described as being like fire, and the storm at the heart of the sun. Fire can be a wonderful thing to a tribe trapped in the dark and the cold. But as Old Mother, and later Martha Jones, warn: ‘Fire will kill us all in the end.’

The TARDIS crew’s attempt to rescue the Doctor ends in failure, although it gives the Doctor his first opportunity to be a hero, threatening to withhold the secret of fire if Ian is hurt. It’s the first sign that the way to influence him is to threaten his companions. The episode ends with the crew imprisoned for the first time, and probably the grimmest, in a charnel house of shattered bones. The Doctor apologises, desperately sorry for the mess he’s got them into. Considering its focus on cavemen, this is a surprisingly subtle, sophisticated and interesting episode.

Next episode: The Forest of Fear