In Connie Willis' future world, time travel is possible, but it isn't profitable so corporations have abandoned it, and instead time travel becomes the realm of the historian. But there are catches. Nothing important can be brought back from the past and time travellers are simply not able to visit events of historical importance. The universe seems to have a way of protecting history - changes to the past invariably get fixed, and travellers that might interfere in critical events find themselves unable to visit them. So you couldn't go back in time to assassinate Hitler - you'd find yourself in Berlin at the wrong time, or hundreds of miles away at the right time.
This is the background to what initially seems to be two parallel stories. Ned Henry, a historian of the 20th century, is actually sent back to Victorian times to try and fix a problem caused by another historian. Simultaneously a rich philanthropist who is trying to rebuild Coventry cathedral as it was immediately before its was destroyed by Nazi bombs in World War Two. The two stories turn out to be closely linked as Ned's journey back inadvertently messes up the time lines, potentially changing the future.
It would be foolish to try and summarise the plot here - what I think that readers should do is to dig out the novel themselves and read it. In places it is hilariously funny, particularly in its depictions of the rigid class structured lives of the Victorians and their strange habits. But what really struck me is how clever Willis' plot is. Everything is tied together very satisfyingly at the end with the author never losing track of the multiple timelines and consequences of change. The detail is excellent - almost everything is significant and Willis brilliantly evokes the Victorian era, the Blitz and a university time-travel department run by a cash starved bureaucracy. I also liked the fact that Willis clearly has thought through time travel - our hero (and the reader incidentally) is unable understand the old English spoken when he travels briefly back to the building of Coventry cathedral.
This is a cracking read and I look forward to Willis' other works.