This story sits slightly off to one side of the Doctor for much of the action.
While it follows on from where Lucie and Alex leave the Doctor and Susan to go travelling, the Doctor has since had his own adventure in Prisoner of the Sun. At the end of that story, the Doctor gets a transmission from Lucie.
But for much of this story, Lucie is struggling to cope with what is happening around her (and Earth) without having the Doctor's assistance, or even presence. The story moves between narration from Lucie, recording the drama for the Doctor's later hearing (assuming he gets it), and the action itself. The story takes place over two years, so there is room for a lot of action to take place.
Old enemies and older enemies return to interfere in Earth's history, and Lucie, Susan and the Earth survivors struggle to thwart their plans.
I really liked this story; it had all the elements that make a great Doctor Who story (except, for much of it, the Doctor!). But it had the `classic' feel, which I really love about these type of stories - Daleks, other enemies, Susan, struggling to survive on Earth ... and a great cliffhanger!
Only grumble I keep thinking is that it would have been better to package the next story, To the Death, with this one - after all, they are interdependent - how many people would want one and not the other? Seems rather like cutting a novel in half to package them separately like this.