Scipio Hostus, prefect of Fort Aesica on Hadrian's Wall and a devoted Roman officer sent to pacify Britain's northernmost frontier, leads a small patrol of ragged limitanei in pursuit of Picts. After losing two men to a Pictish ambush, Scipio's company routs the raiders and continues to Dogtown, a half-domesticated Pictish village, where they bully the inhabitants and rest. Scipio rides off alone, missing his wife Galla and his children Lucia and Linus back in Rome, and reflects on Caledonia as the one place his civilizing mission cannot tame.

In an empty field he discovers a mound, climbs it, then finds a hidden door opening into a chamber where a tall, pale woman with indigo tattooed eyes sits doing metalwork. Though he intends to take her hostage, he is enchanted: he sweeps her floor, chops wood, and drudges for her until he wakes outside, his horse gone. He returns to Aesica to discover three weeks have passed.

Back at command, Scipio doubles down on his duties as Britain's situation deteriorates: barbarian pressure mounts, Rome strips troops for continental wars, and his letters go unanswered. His optio Servius and a squad vanish; Dogtown is found deserted and ringed with skulls, so he burns it. In November, the Dux Britanniarum's orders finally arrive: return to Rome, the cohort is disbanded, Britain is abandoned.

Enraged, Scipio rides north into Caledonia, kills three Pictish hunters, and finds the mound again, now doorless. He kneels weeping and praying until the woman appears. This time she beats him and enslaves him in earnest; he serves her and her uncanny guests for an indeterminate time. When he finally rebels and tries to stab her, his blade breaks. She reveals she predates Romans, Picts, and fairies, tells him Rome is being sacked by Visigoths, and brands his face with a hot knife while teaching him that walls mean death and empires cannot love.

He wakes outside, freed from Britannia's spell, and runs south intending to sail home to Galla. But Aesica is in ruins, the Wall a broken spine: a hundred years have passed, Rome has fallen, and everyone he loved is dead. He eventually drifts south to serve a new king, though he never joins the Round Table, becoming the famed Sir Scipio who fights as though indifferent to life or death.