The Roman Republic is under attack on several fronts; while a dangerous rebellion in Italy is being crushed by Sulla, the armies of Mithridates the Great are sweeping through Asia Minor.

The young Gordianus is living in Alexandria when he receives a message telling him of the plight of his former mentor Antipater of Sidon at the court of Mithridates in Ephesus.

Accompanied by his beautiful slave Bethesda he travels incognito to Ephesus to help Antipater, even though they parted on bad terms, in spite of the great danger that threatens all Roman citizens under the rule of Mithridates.

Together with the Jewish spy Samson, Gordianus must try to stop a ritual sacrifice in the hope of thwarting the King's plans to have all the Romans in the lands under his sway be massacred.

Although Gordianus and his co-conspirators foil the ritual sacrifice, they are not able to prevent the massacre, nor do they even warn the Romans of this impending action that has gone down in history as the Asiatic Vespers.