When a pirate radio ship disappears, the Navy investigates. When the Navy submarine disappears, Lethbridge-Stewart knows something is up. Deep in the sea an alien race known as the Ynir are preparing plans for their 'grandfathers', a species of intelligent killer plants, to wipe out all life on Earth and take the planet over.
This is the book in the series that really captures the era well. It feels like a James Bond movie but with killer plants akin to The Day of the Triffids or Doctor Who's own Seeds of Doom. At the same time it fits in well with the previous six books and Doctor Who in general. Unlike previous books the threat is world-level here and it's interesting to see that additional tension and the way the team deals with it- they are starting to get used to this sort of thing. There's also a great link to the First Doctor story Planet of Giants which I thought was fantastic.
Peel's other strength in this book was the characterisation. For a start he deals with the main cast really well, making Lethbridge-Stewart and Anne Travers feel just like do on screen. He also introduced us to some great one-off characters like pirate radio DJ Mary. The thing that really surprised me was his excellent characterisation of the aliens. So often the idea that aliens have some sort of personality is ignored but Peel addresses it really well and in doing so creates a more believable threat.
My one issue with this book was the ending. It felt a little rushed and much of the solution came from a frustrating deus ex machina, albeit a previously established one. As much as I thought this was clever it felt a little frustrating in terms of the story.
Overall though a stunning read and this series continues to reach a high standard.