It's September 3rd, 1939, the exact moment when Chamberlain, the UK's Prime Minister, addresses the nation and tells them that Britain is officially declaring war with the Nazi regime. At the same time, a Secret Service operative sneaks into Maisie's apartment and waits for her there. The thing is - Dr. Thomas needs her to do something of the utmost importance to the Queen.
She is to track and locate the murderer of a gentleman who fled Belgium when it was occupied by the enemy. That happened about 23 years ago when the Great War was raging. London looks like it's getting ready for destruction, and that's exactly how the government and the simple folks feel. Bomb shelters are on every corner, and the enemy could be coming from any direction at any moment. Soon, another refugee from Belgium is found dead in the city.
So, Maisie starts to dig deeper and deeper into this mysterious case, and that's when she encounters a refugee from UK's capital: there's a little girl standing at her doorstep, and she cannot utter a single word. Nobody knows who she is, who her parents are and what she's doing all the way over in Kent. The only thing that they know is that she needs somebody to look after her and that her name is Anna.
Maisie's search for answers takes her to all kinds of dangerous places, and the stakes get higher by every passing moment. Britain is trying to hold it together and to deliver a deadly blow to the enemy. The gravest hours are approaching rapidly, and it's time to get down on one's knees and pray. In This Grave Hour is a top-notch British historical mystery/thriller with superb writing, great historical accuracy, and quite a gripping plot.