The core of this story is that of a group from way in the future who travel way back to the past and muck about with the accepted history of Earth. When a young Neanderthal man finds his way to 2005, the Doctor, Rose and Jack must try to put things to right. So far, so good.
I found this story problematic though. Firstly, Jack is separated from the Doctor and Rose and plays virtually no part in the story at all. The premise on which Das, the Neaderthal is not able to return to his own time is overplayed to the point of parody as he finds a new life in the modern world. Really didn't gel I didn't think. Meanwhile, back in 28,000 BC or thereabouts, there's a world where they go hunting on Monday, Tuesday and Thursday and get Wednesday off, and they have marriage ceremonies which include the Great Fish of Matrimony. Sorry, don't buy it.
The futuristic part of the story is good. The mad scientist part of the story is good. The Neanderthal part of the story is bad. Pity, really. Even the characterisations of the Doctor and Rose don't win the story back to being all that great - I didn't feel that the Doctor was really true to his Ninth Doctor self; he seemed more like the David Tennant Tenth Doctor. This story was picked to be representative of the Ninth Doctor in the 50th anniversary celebration series of novelisation; I am sure there must have been better stories out there for the purpose.