Dan Wilson is an agent with British Intelligence.
The year is 1942. The Second World War is well underway. In England, British Intelligence has discovered a German agent secreted at Lynchester School, a prep school somewhere in the east of England. Learning that the enemy operative, named Wilhelm Miers, was set to meet with another agent to receive some classified documents, it is decided to ask fellow student Dan Wilson, who happens to have a passing resemblance of the captured spy, to take his place for that transfer.
What sounded like a simple, short impersonation turned pear-shaped quickly when British soldiers, not in on the plan, get curious about the late night rendezvous and a chase ensues. A stray bullet grazes young Wilson and he soon loses consciousness. The enemy agent he was meeting carries him to a nearby plane he planned to use to return to Germany. Now there is an extra passenger and before he knows it, Wilson is landing in Germany.
Three items will work together to change Wilson's frightening prospects of long imprisonment as a British spy.
The first is that Wilson had lived for many years in Berlin before the War. His father had been a prominent British foreign correspondent assigned to that capital and young Wilson went with him. As a result, Wilson not only speaks German as a native, he knows and understands Berlin quite well.
The second is that being 17 years old and already nearly fully grown, albeit in a tall, spindly manner, mistaking him for a German undercover operative a couple years older is not a stretch, especially since young Wilson looks like a blond Aryan.
The third and last is that the current head of the German Espionage Organization, to which the faux returned agent is assigned, has never met Wilson before so believes the young man standing before him is the real deal.
So it is that for a short time, Dan Wilson, formerly a soon-to-graduate student, is now an undercover operative inside enemy lines.