York, 211 AD. Following the death of Roman Emperor Septimius Severus, his sons Caracalla and Geta share the imperial crown, and immediately begin plotting each other’s murder. Whilst among those returning to Rome with the young Caesars is Marcus Granua, imperial adviser and philosopher, who some say, has more right to be emperor than either of them.
Meanwhile a series of prostitute murders exemplifies the degeneracy at the heart of the empire. The imperial police have little interest in protecting whores, so investigation falls to Quintus Celer, a former charioteer lamed in an accident, who has to deal both with thugs from the Roman underworld and the equally corrupt and even more dangerous inhabitants of the imperial palace.
The intervening years have not been kind to the friends Marcus left in Rome, indeed fatal to one of them. Of the two women who had captured his heart, one has become a dirty joke, the other a spiteful featherbrain, with a disastrous marriage. And could her studious, boring husband be the secret serial killer? Truly, Marcus wonders, was the Cynic Diogenes right when he wandered the streets of his home city with a lantern at noonday, ‘looking for an honest man’?