Geoffrey Mildmay is an agent with British Intelligence.
Exactly which department and what his duties are remain a mystery, something the quite narcissistic Mildmay would delight in. He is a dashing, brave, adventurous rogue who will let nothing stand between himself and duty to his country, unless a pretty girl beckons. In more modern vernacular, he would be called a "playa" and would definitely deserve it. He likes his spirits, he enjoys gambling and sports and lounging about on a warm afternoon with a lovely lady who dotes on him. He also enjoys adventure and a spot of danger.
The three adventures of Mildmay take place near the end of WWII and the peace that followed but which brought in the Cold War. The tales are all narrated by an American contemporary of Mildmay, Bill Stacy, who knew him, and sometimes liked him, when they both attended school together. A few years later, they were reunited for the first mission which needed the engineering skill of Stacy and the piloting skill of Mildmay.
After that adventure, the two men went their own way but were brought together again by chance and thrown into another mission. Much the same took place for the third and final assignment.
The books are as much about the "frenemy" relationship between these two men, especially as it pertains to the various women that they both fall for, with Stacy's affections being usually unrequited and Mildmay's being constantly feckless, as it is about the dangers that they do legitimately face.