The books follow the lives of two officers in the Imperial Roman army, Quintus Licinius Cato and Lucius Cornelius Macro.

The first book, Under the Eagle, concerns the induction of Cato, his transition from imperial slave to optio (junior officer) and the lifelong friendship he forges with Centurion Macro.

The sixth book, The Eagle's Prophecy, opens with the two on leave in the city of Rome and detailed to carry out a pursuit of pirates operating in the Mediterranean.

The seventh and eighth books, The Eagle in the Sand and Centurion, take place in the Roman Empire's eastern provinces, Judaea and Palmyra respectively.

While returning to Rome from Palmyra, the protagonists are shipwrecked on the island of Crete in The Gladiator, which leads to them being sent to Egypt in the tenth novel, The Legion.

The series also features Vespasian and Vitellius as supporting characters, during the formative years of their careers, and foreshadowing their future rivalry for the Imperial throne.

This bond deepens when Macro confesses that he is illiterate and asks Cato's help in learning to read, and so maintain his position as an officer.

Having been born a slave and the property of the state, he was given an opportunity by the Emperor as a favour to Cato's late father to enlist in the legions and be given his freedom.

Cato has lived a relatively luxurious life as a slave within the Imperial palace in comparison with the rank and file of the legions, and after accepting the Emperor's offer he joins the Second Augustan as Macro's Optio.

In the first novel, he is only sixteen years of age, tall and gawky, and so weak-looking that many of the officers, including Macro, place bets on how long it will be before he quits or is killed.

The Emperor grants Cato an immediate commission as a Centurion, but because of his age Vespasian makes him an optio as a compromise, which causes the officers and his fellow recruits to resent him further.

While traveling back to Rome, she and her father are shipwrecked on Crete with Macro and Cato, where she is captured by the rebel leader Ajax, but manages to escape.

Cato learns of Julia's unfaithfulness upon his return to Rome in Invictus, however in "Days of the Caesars" she is revealed to have been faithful by Domitia (Vespasian's wife), using her supposed infidelity as a cover to raise funds on behalf of Britannicus.

Freed when a massive earthquake devastates the island in The Gladiator, he leads an army of other escaped slaves and captures Macro and Julia but is defeated by Cato and his troops.

The reason is unknown but some speculate that it has been done in effort to make the books accessible to a wider audience, and it also explains the change in cover illustrations.