"The Gabinian Affair" presents the memoir written by a retired Roman soldier, Gaius Marius Insubrecus, who served Caesar during his conquest of Gaul and in the subsequent civil wars. He later served under Caesar's son and heir, Octavianus, in his war against Marcus Antonius and Cleopatra.

As a youth, Insubrecus was caught between two worlds. He lived in the mythical tales told to him by his grandfather about the heroic past of his people, the Gah'el. However, his mother, Valeria, was determined to make a practical and successful Roman gentlemen out of him. On top of all this, he fell in love with Gabinia, the beautiful daughter of a Roman Senator, whose family was determined to kill him to uphold their honor.

Insubrecus tries to escape the assassins sent after him from Rome by hiding in the Roman army, right at the time that the new governor, Gaius Iulius Caesar, launches his legions into the forests of Gaul to stop an invasion by a fierce and ruthless tribe called the Helvetii. Insubrecus is plunged into a world of violence, intrigue and betrayal, as he tries to serve his new patron, Caesar, and to stay alive, while pursued by Roman cutthroats and Gallic warriors.