All Clear - Connie Willis

Oxford historians Michael Davies, Merope Ward and Polly Churchill have been sent back in time to observe different aspects of World War II. Mike is observing heroism at Dunkirk; Merope is studying London evacuees; Polly is posing as a shopgirl in one of the big Oxford Street department stores

But something has gone awry, and all three have found themselves unexpectedly trapped in 1940. They’re struggling not only. to find their way home to 2060, but to survive as Hitler’s bombers attempt to pummel London into bloody submission.

That’s not the worst: they are all trying to pretend they’ve not noticed little discrepancies in the historical record, because discrepancies would mean that one or all of them have affected the past, possibly even changing the outcome of the war. Everyone knows that the past can be observed but never altered – but suddenly it’s looking like that theory is horribly, tragically wrong.

And back in 2060 Oxford, Mr Dunworthy and Colin Templer are engaged in a frantic, impossible struggle of their own: to find three missing needles in the haystack of history.