Karen is on holiday at the seaside in twentieth century England. She finds a bronze mirror, green with age, almost buried in the sand. Looking into it, she is transported back in time to the Roman Empire. She encounters a Roman officer who mistakes her for a runaway slave. She is sold several times, ending up in a wealthy household in Nero's Rome, as nursemaid to the children of the family. She falls in love with a Greek slave boy, witnesses gladiatorial games, and joins a group of early Christians. When the Great Fire of Rome comes, she is forced to flee from persecution, and hides in the catacombs. She faces many hair-raising adventures in her attempts to return to her own time.