Shark is an agent for British Intelligence.
Obviously that solo identifier is just the codename for the actual operative.
The activities of this agent take place in 1940 in the Channel Islands, an archipelago in the English Channel comprised of two large and several smaller islands which have been independent for centuries but which fall under the protectorate of the United Kingdom.
In June of that year, the British withdrew, certain that the German occupation was a foregone conclusion, and much of the islands fled as well. A good number remained, though, and thus fell under the control of the invading forces one month later.
On one of the islands, likely one of the two larger ones, either Jersey or Guernsey though which one is never specifically mentioned, a school for boys operated. Most of the instructors and a good number of the students had left during the evacuation but one teacher and a handful of students remain. While that teacher is a die-hard, swastika-armband wearing German follower preaching adherance to the new order and happy to teach German to the boys, several of the students remain defiant.
Four in particular play a major role in the activities of Shark against the occupiers. They are: Lenny Francis, Micky Greaves, Terry Trent, and Chipper Price. All are teenagers but none likely older that 14 or 15.
As one of them mentions when they meet right after the Germans assumed control, "our fathers are all in the Services fighting the Germans and I reckon we should do the same!" They are certain they will be able to accomplish things because "we know this town far better than that slob [Kolonel] Kranz and his goons" and know how to move about the town without being spotted.
When they make their first act of defiance, they will encounter a hooded figure who saves their bacon and then tells them that they can help him in his fight against the Germans but only if they act only when he tells them and does what he commands.
Over the course of their adventures they will be saved by the Shark a couple of times and will in turn help him succeed. Shark is the operative in charge but the kids are really the stars.