Kivrin is a student historian sent back in time to December 1320 to observe a medieval Christmas in an Ox­ford­shire village. Back in the Oxford of the mid-twen­ty­first century, her tutor Dunworthy grows extremely worried, as the tech who sent her back collapsed into a coma, mumbling something about slippage.

The book alternates between Kivrin and Dunworthy. Kivrin falls sick just after she lands. She wakes in an isolated, snowbound country manor, being nursed by Lady Eliwys and her mother-in-law Lady Imeyne.

Dunworthy becomes ever more worried when Oxford and its environs are quar­an­tined. The comatose tech has an unfamiliar virus, which starts spreading.

Kivrin becomes obsessed with finding her way back to the rendezvous point within the next two weeks, or she’ll never go home. She ends up looking after Eliwys’s two daughters, Rosemund and Agnes. At Christmas, people start falling sick and dying. She learns that she’s actually in 1348, the middle of the Black Death.

Back in the future, people are dying all around Dunworthy, who now stands in loco parentis to twelve-year-old Colin. A plague is loose in Oxford too.

The details of time travel inform some of the plot, but Willis con­cen­trates on weaving two parallel tales with eerie sim­i­lar­i­ties. The future Oxonians are beleagured, but far better able to cope, emo­tion­al­ly and medically. Kivrin despairs as the Ox­ford­shire villagers die all around her. She un­der­stands the mechanics of the plague, but is helpless to address it without modern medicine.