On a journey from the Kingdom of Versailles to Marseille with her parents along with a local named Jacques, Annette and Jacques are captured by slavers and separated from the caravan. The two are taken to an even more primitive alternate by renegade Crosstimers who use the Crosstime technology to run a slaving world.
The novel details Klein's attempts to escape back to her home timeline to unmask the illegal activities.
Through Darkest Europe depicts a world where the Islamic world is more progressive and enlightened than ChristianEurope, with the POD being a series of philosophical "reversals" in the 12th and 13th centuries.