The story follows them as they head to modern day London due to the TARDIS detecting some kind of time disturbance. Before long they find out that a Neanderthal man named Das has somehow been plucked out of his own time trapped there (they can't bring him home in the TARDIS). So, the Doctor and Rose head back into the past in order to find out what it is that may have sent Das back, and Jack remains in the present day as he tries to help Das to adapt to modern life.
I'm actually a little disappointed with how Jack is used in this novel. Jack is one of my favourite of the Doctor's companions, so I was quite excited that he was in this book, but the fact that he stays behind with Das means that he pretty much entirely removed from the main adventure. Nonetheless, there are still chapters about Jack and Das and they are very enjoyable, I just wish Jack could have been more involved in the story.
The Doctor and Rose, meanwhile, find themselves thousands of years in the past, where a colony of humans have travelled back from the future to find a new home. This society is quite different to the people of today, and something very strange is happening, but I shan't spoil it. It's interesting to compare the cave people, modern humans and future humans.
The story is written with a very light tone, and while there are some very serious moments, even they are given a humorous edge which often relieves tension. Having said that, I did find that the presence of humour is the darker scenes actually helped to make the novel seem somewhat eerie, and even discomforting at times. I do think that it is occasionally TOO light though and could have done with a slightly bigger sense of seriousness.