The Doctor takes the TARDIS to the village of Little Hodcombe, in order to visit Tegan's grandfather. Initially it appears that they have arrived in the wrong time period, but in fact there is an historical reenactment of a battle during the English Civil War being carried out. But there is something more at work behind the scenes, and Tegan's grandfather is missing...
Based on a 2 episode TV serial by the scriptwriter Eric Pringle, this book seems less rushed than the original series (which probably should have been 3 episodes long). With a little more time and room, the story is better in this form than the televised version.
Unfortunately, the story itself has some problems not uncommon in Doctor Who. The Doctor has two companions, Tegan and Turlough, who each end up with not much to do. Instead, some supporting characters that feature in this story get to do all the companion-type things. One would have hoped that the story could have been written a little differently to allow the regulars larger roles.
However, the idea behind the story is quite a good one, so this particular failing is probably something more likely to be seen in retrospect rather than as the story progresses.