Thessalonica tells the story of the siege of the Christian city by invading Slavs and Avars in the 6th century. The novel's protagonist, George, is a simple shoemaker who lives in the city and obtains the aid of several survivors of the endangered Greek pantheon, including centaurs and satyrs, to assist the Christian God and saints against the Slavic gods which command their own supernatural beings. Within days the city is surrounded, and though the strength of the Christian God enables the population to keep the barbarian hordes at bay, George realizes they may need more to defeat their enemies. With the help of a pious but broad-minded priest, George convinces the satyrs, centaurs and others to save the city.