On the eve of Solstice, Cait, a young woman belonging to Britannia's Cantii tribe, awaits a rite of passage that separates girls from women. However, 400 ships, carrying 20,000 Roman soldiers, land on the shores close by, under the command of General Aulus Plautius, to take Britannia in the name of Emperor Claudius. Cait's initiation into womanhood is interrupted by a Roman attack on her village, leaving her "betwixt" – neither girl nor woman – and everything she loves left in ashes. She is only left alive thanks to the actions of an outcast Druid, Divis, who has had unsettling visions of what is yet to come. Meanwhile, 40 miles inland, the border between the Cantii and Regni tribes is the site of constant bloodshed. A marriage, brokered by Cantii prince Phelan, takes place as an attempt at peace between the warring factions. But peace is a rare thing in Britannia, and this is no exception.