One of Diana Wynne Jones' favourite tropes seems to be time-travel, and she also seems fond of creating characters who are not what they seem. In Hexwood she does her best to push both these ideas to their limits. The book opens with teenager Ann Stavely lying in bed with a nasty virus and watching the mysterious arrivals at Hexwood Estate, a short distance down the road from the greengrocery shop/flat where she lives with her parents and brother, Martin. When she recovers sufficiently from the virus to go out, she goes down to Hexwood, hoping to find out what has happened to the visitors who've arrived yet never left. In the wood she meets a man named Mordion who tells her that he has been trapped in stass-sleep (a form of suspended animation) for thousands of years, after he tried to disobey the Reigners. Using magic and blood from himself and Ann, he creates a boy whom he intends to grow up to be a hero to go after the Reigners on his behalf. Mordion explains to Ann that Hexwood is being affected by something which is creating a set of "paratypical extensions", which is akin to casting a spell. Together Ann and Mordion find out that the field of effect is controlled by something called a 'bannus',. which is effectively an ancient but powerful computer. It cause time to repeat, jump forwards or backwards, and do other odd things. The bannus field also spreads outwards from the farmhouse on to the nearby housing estate (on which Ann lives), and pulls in people and objects which it then manipulates.
I won't go into any further detail about the plot which is complex and extensive. I don't want to risk putting anyone off by summarising poorly. None of the characters in the book are who they appear to be, not even the robot Yam (short for Yamaha), and not all of them are actually natives of Earth. Suffice it to say that this complex and detailed tale needs to be read with one's full attention, or the different names applied to the same people will confuse the reader. This is definitely a book for older readers. I did enjoy it, and I would probably re-read it in the future.
Sidenote: I've included a cover-image of the book, because I think it gives an idea of the potentially sinister nature of Hexwood and the people who live in it. Note how the arist has made a face out of the branches, and there are various faces on the branches, which will be clearer on this large image.