The titular character is no ordinary sea dog; educated at Oxford University, he lives incognito as a clergyman, respected in his parish as a gentle and affable man of faith. However, circumstances and revelations unveil his shocking past as a pirate and smuggler, with the resulting ructions a catalyst for the scrapes which follow.

The story of Doctor Syn has some basis in reality; the high taxation which France levied upon luxury goods such as alcoholic spirits and tobacco in the 18^th century led to a lively and active smuggling culture across the English Channel. The trade in untaxed products was so lucrative that local militias and even detachments of Kent's army facilitated its continuation. It is with a combination of quick-wittedness and guile that Syn originally gained expertise at the pirating life; in equal measure, he uses such social gifts to lay low long term as a humble vicar.

This book is but the first in a classic series of adventure books featuring Doctor Syn, a character riven with complexity who made Russell Thorndike one of the most popular fiction authors of the early 20^th century.