Young American scholar Charles Bascombe is thrilled when a well-known Cornish bibliophile invites him to Cornwall to see a mysterious medieval manuscript he has recently come to possess. Accompanied by his Oxford mentor, Professor William Wentworth, Charles goes to the Lizard to study the manuscript. He soon determines that it may contain a long-lost poem by fourteenth-century poet Geoffrey Chaucer. After Charles has transcribed the work, the manuscript, along with the professor's car, is stolen. A series of adventures ensure, as Charles and the professor attempt to recover the manuscript, including seductions attempts by alluring women, Charles being stalked at night in an eerie churchyard, and a physical altercation in an alleyway behind a pub, and a vicarage fire. Ultimately the manuscript is recovered. But does it really contain Chaucer's “Book of the Lion”? Perhaps the key lesson that Charles learns is that Cornish mermaids pose a great danger to mortal men.