In an alternate 1914 Europe, fifteen-year-old Austrian Prince Alek, on the run from the Clanker Powers who are attempting to take over the globe using mechanical machinery, forms an uneasy alliance with Deryn who, disguised as a boy to join the British Air Service, is learning to fly genetically-engineered beasts.

Polly's visit to her grandparents in Connecticut becomes an extraordinary experience as she encounters old friends and mysterious strangers and finds herself traveling back in time to play a crucial role in a prehistoric confrontation.

Sixteen-year-old Gwyneth discovers that she, rather than her well-prepared cousin, carries a time-travel gene, and soon she is journeying with Gideon, who shares the gift, through historical London trying to discover whom they can trust.