In the future, time travel is possible. Much of its basic premise is still theoretical, but enough is understood that we're able to put it to use, to travel backwards in time to embark on various projects in historical research. It was learned early on that history couldn't be actually changed , and so time travel lost its appeal for most people. Time travel still has its uses, of course, as in the case of Lady Schrapnell. You see, an ancestor of hers had a life-changing experience during a visit to the cathedral, when she looked upon a little item known as the bishop's bird stump.
History indicates that the item disappeared or was destroyed during the raid.
She wants to send a group of historians back to the time of the raid, to find out what happened to it, so that they might be able to retrieve it. He's in desperate need of a vacation. He doesn't even know what the object is, only that it must be one of the things in his rather large collection of luggage. Or maybe it's not. Layer upon layer of mystery is added, complete with all of the paradoxical situations that can arise through time travel.