The frontier province of Bithynia is a cesspit of sedition, rotten with corruption and seething with hatred of Rome. When a rich Roman official is found dead on a desolate hillside; two riderless horses tethered in a nearby wood, it is assumed he was killed as a protest against Emperor Trajan's unlimited power. But Pliny the Younger, newly appointed Governor of Bithynia, is not so sure. Who was the other rider? What were the two of them doing in the middle of nowhere? And what links this murder to a secret cult of the Persian sun god, Mithras - the Bull Slayer?