Oswald de Lacy, sent to live in a monastery when he was 7 years old, returns home to become Lord of Somerhill Manor after his father and brothers succumb to the plague. It's the mid-1300s, and being head of the plague-afflicted manor isn't easy. Many of the estate's rent-paying tenants died and their dilapidated homes are falling apart; there are barely enough workers to harvest the manor's crops; the self-serving village priest, John of Cornwall, encourages superstition and sells fake religious relics; Lacy's mother is domineering and his unmarried sister Clemence is difficult; and to top it off a young girl, Alison Starvecrow, has been found dead in the woods. Moreover, the dead girl visited Somerhall Manor looking for Oswald shortly before she was murdered.
Because the plague killed the village constable, it falls to Oswald to look into Alison's death. However Oswald is seriously unequipped for the task. He's only 18 years old, has been sheltered for most of his life, and knows much more about the bible and Greek literature than everyday life. Neverthless, Oswald's mentor, elderly Brother Peter, convinces the boy to investigate Alison's demise. This is made more difficult by Priest John, who claims that satanic dog-headed men killed the girl. The priest organizes prayer sessions and relic sales to ward off the creatures - coincidentally drawing the villagers away from their much-needed work in the fields.
Oswald looks into Alison's death, concludes she was murdered, and promises to find the killer. And matters escalate even further when another girl disappears. Oswald looks for clues, questions witnesses, and tries to find viable suspects - but criminal investigations in the 1300s were pretty primitive. (Note: I can't help but think a lot of people literally got away with murder in those days.)
While all this is going on, Oswald's sister Clemence becomes engaged to a neighbor, a bad-natured, previously married wife-abuser who hopes to take over Somerhill Manor by any means. As a result Oswald's life is endangered, he has some dreadful adventures, and he meets a boy with terrible birth defects. Oswald also has to deal with his mother - who tries to get him betrothed to a dull young cousin he doesn't like, and with the shocking (to him) discovery that his father left a dozen or so illegitimate children scattered around the village.
I think the book succeeds pretty well as a historical novel, giving the reader a little taste of life in England in the Middle Ages: the feudal system, the arranged marriages, the weddings, the jails, the courts, and so on. It's also an okay suspense story, and Oswald does solve the crimes in the end. I prefer modern mysteries but I think fans of historical mysteries would enjoy this book.