Merlin has been relegated to magical research and teaching towards the tail end of WWII. His superiors feel that allowing him to put himself at risk is not a good idea. They fear the blow to the morale of the magical troops if anything untoward should occur to him.
When Nimue, and Merlin's unborn child, come under magical attack he must travel into the heart of enemy territory to save them, while trying not to violate his orders.
If he fails, his wife and child will either perish or be possessed by an old enemy. One who would love nothing more than to gain herself a new body while causing anguish or death to Merlin. If he succeeds, his wife and child will be safe and the war may be brought to a close sooner than anticipated.
Merlin has no idea exactly how he'll achieve this task but he knows that he cannot live with himself if he doesn't.