Alistair and Freddie Valentine are agents with British Intelligence.
Alistair Valentine is the operative with by far the greater experience as he was years ago recruited by Nicholas Brimblecombe, a former professor and now "a senior member of British Intelligence and 'the ultimate spymaster'". Freddie is far newer to the clandestine world, only being snatched up by the Professor when an injury brought an end to Freddie's impressive RAF piloting career.
The time period for the activities of the Valentine brothers is the Second World War and the first couple of years after the cessation of hostilities, though there are a good number of flashbacks to the years leading up to that horrendous conflict.
The two Valentines, though close personally, are quite a bit different in temperament and experience.
Alistair is a trained polyglot who before the War specialized in the buying and selling of fine European wines. His grasp of that field of knowledge plus his abilities to talk like a native in several languages makes him a perfect recruit by Brimblecombe long before actual hostilities began.
Freddie is a man of action and excitement and seemingly more at home in the air piloting different aircraft than on land. He would become an ace as a fighter pilot proving his prowess in the skies up until an injury would curtail that. At a loss for what to do next with himself and still eager to serve, he is readily receptive to the request by Brimblecombe to come into the cloak and dagger world.
And playing major roles in the adventures of the Valentines are two women who match (and exceed) the skillset and daring of these two male operative. Joselyn Foster, American, and Lucy Fry, British, will each be brought into action and will prove that they quite capable of facing the same sort of dangers as the Valentines, and more.