The book opens immediately after the events of Under the Eagle, with the troops relaxing and watching prisoners of war fight to the death in a makeshift arena.

Meanwhile, the legate of the Second Legion, Vespasian, worries about his wife Flavia back in Rome, whom he has learned has connections to "The Liberators", a group of conspirators who want the feeble-minded Emperor Claudius out of power.

As the Britons under Caratacus have heavily fortified the opposite bank, Macro and Cato are ordered to scout ahead for a ford upstream.

The Ninth Legion, supported by artillery fire from triremes on the river, crosses and assault the enemy ramparts.

Using the ford upstream, the legionaries are able to surprise the Britons and attack them from behind, overrunning their encampment; Cato is badly burned when he accidentally spills a cauldron of boiling water over himself.

All thought of it is put out of his mind, however, when Claudius arrives, escorted by the Praetorian Guard and elephants to overawe the Britons.

When it is rolled up in a certain way, the markings become a coded message; Cato is only able to discover that it concerns a plot to kill the emperor before he is knocked out by Lavinia, who read on ahead and saw Vitellius' name.

Vespasian is tasked with an independent command for the coming months; clearing the south bank of the Thames of resistance.

As for Cato, he is distracted by Macro, who promises to introduce him to his latest conquest - a young Briton called Boudicca...