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Although it takes a healthy suspension of disbelief in parts (oh, the conveniences), it's overall a pretty charming read. Telling two parallel stories - one of a contemporary girl in London, one of boy who was evacuated to the Lake District during World War II - I admit that I teared up a little.
It was pretty average until the end, where it got a bit sentimental and precious. And then everything was right in the world for everyone forever. Can't complain too much, since before that it was just meh. The story was ok, but the way it was written made even the more exciting scenes flat.