The Wicked Day is a sequel to Mary Stewart's hugely acclaimed and hugely successful (and, indeed, hugely enjoyable) Merlin Trilogy. Here, the main character is not Merlin, but Mordred - King Arthur's illegitimate son with his half-sister, Morguase.
As in the Merlin Trilogy, Stewart battles to give us a story that might have happened. She links what little history she believes is true to the copious and varied legends of the Arthur legend. And she does it, for the most part, excellently.
The story itself overlaps with the Merlin Trilogy - it picks up with Mordred as a young child, and follows him onwards through his life in and around Arthur's court, and the adventures that follows. Unlike the Merlin Trilogy, this book is written in the third person; following Mordred and his story, but not told from his point of view.
There is very little mention of Merlin himself, and the story is totally self-contained.
I really enjoyed the book; it is convincingly written, linked in enough with history to let you believe. I must admit that the ending of this book is a slight anti-climax as though at that point - where Stewart had the least definite facts, and the most varied legends - she lost her way slightly and was unsure what to write. But that is only compared to some of the stunning climaxes and situations that are crafted throughout her writing.
Overall, this is an excellent read and a must, really, for all fans of the Merlin books.