All Clear
Connie Willis

Oxford historians Michael Davies, Merope Ward and Polly Churchill have been sent back in time to observe World War II. But something has gone awry, and all three have found themselves trapped in 1940. They're struggling not only to find their way home to 2060, but to survive as Hitler's bombers try to pummel London into bloody submission. That's not the worst : they are all trying to ignore little discrepancies in the historical record, because discrepancies mean that one or all of them have altered the past, possibly even changing the outcome of the war. Everyone knows the past can be observed but never altered - but suddenly that theory is looking horribly, tragically wrong...