In the garden of a country estate, an ancient monument holds a cryptic secret. Chiseled on it is a coded inscription that has baffled the world's cleverest minds for two centuries. When a child playing near the monument stumbles upon the dead body of a man, another mystery is revealed: in his pocket is a scrap of paper bearing a sequence of letters. The police suspect that it may be part of a coded message but their investigation leads nowhere. The case at a standstill, Lawrence Kingston, retired professor of botany, is hired to conduct an independent inquiry. Soon, Kingston finds himself swept along in a dangerous undertow of a centuries-old family feud, a suspicious poisoning and veiled threats, leading him to fear for his own life. To solve the secret of the past and crimes of the present, he must decipher a complex code hidden in the walls of an old manor house. But to do so, he must first delve into the minds of three eminent 18th century Englishmen to fathom what part they played in the age-old mystery. As his search for the truth narrow, his worst fears materialize when he becomes the next target.