With her trademark understated, eloquent style, Willis expands the conceit of her Hugo and Nebula winning 1982 story Fire Watch into a page-turning thriller, her first novel since 2001's Passage. Three young historians travel from 2060 to early 1940s Britain for firsthand research. As Eileen handles a measles outbreak during the children's evacuation and Polly struggles to work as a London shopgirl, hints of trouble with the time-travel equipment barely register on their radar. Historians aren't supposed to be able to change the course of history, but Mike's actions at Dunkirk may disrupt both the past and the future. Willis uses detail and period language exquisitely well, creating an engaging, exciting tale that cuts off abruptly on the last page. Readers allergic to cliffhangers may want to wait until the second volume comes out in November 2010.