David and Mariella Sheridan are agents of British Intelligence.
When we first meet them they are not a couple though they will become so as time goes by. In fact, this new romance will come despite the fact that with Mariella being deep undercover in multiple identities, she will come to the suspicious attention of David unaware that she was working for the same country's intelligence agencies.
David Sheridan is already an operative when we are introduced to him and his investigations into the activities of Russian agents will take him into contact with Mariella whose suspicious behavior makes him think she might well be undercover herself; unfortunately and mistakenly thinking it is for the other side.
Mariella, for her part, earns this suspicion as she is out to get revenge on the Bolsheviks who have murdered her father, a diplomat for the British government, as well as a couple of her friends. To find some form of justice, she opts to secretly join with British Intelligence and to use her not insignificant acting abilities to take on many parts, such as "masquerade as a low-class Russian servant, an actress, a beautiful courtesan, a sprightly French girl, an old woman", and Agent 'No. 24'".
By the end of the first adventure these two will have fallen in love and decide to settle down to get married, leaving the espionage world behind. Unfortunately, both are quite used to excitement and danger by now so marital bliss is having a hard time filling their home. Both are decidedly bored. When "an international secret service man" named Verlayne, working on behalf of the League of Nations, asks for their help in looking into various matters, both eagerly leap at the offer.