It is October 1477 and Roger the Chapman, newly married and still enjoying wedded bliss in Bristol, England, is surprised to find his old, familiar feeling of restlessness returning. Within a month he is setting off, once again, on the ancient ridge road that dissects Dartmoor and heads for Plymouth, driven by some instinct that he is needed there.

Roger accepts a lift from a carter who is going to visit his daughter, Joanna, in the oldest part of the city. Roger's instinct is soon proven correct when Joanna tells the story of her neighbor, Master Capstick, who was brutally beaten to death. The chief suspect is Capstick's great-nephew, Beric. Master Capstick's housekeeper saw Beric leaving the house that morning, his tunic stained with blood, and many more people saw the young man's wild ride for home on his great black horse. When the King's men arrived at Beric's manor house, though, the horse was already in the stables - and Beric had somehow managed to vanish completely.

The local people, quick to fall back on the witchcraft of their ancestors, blame the Saint John's fern, which if eaten can make a man invisible. Roger, already responsible for solving many difficult mysteries, suspects that there is a more obvious answer and begins his own inquiries. Roger notices that he is not the first to approach witnesses, and when an attempt is made on his life, Roger knows he must be close to a truth that is even more extraordinary than the superstition - if only he can live to tell it.

The Saint John's Fern is the ninth book in Kate Sedley's popular Medieval mystery series featuring Roger the Chapman.