Peter Murphy is an agent with American Intelligence.
Marc Wainwright is a multi-millionaire playboy.
They are two very different people until, in a case of tragic mistaken identity, Wainwright is killed in the street coming out of a bar in Berlin after a night of revelry, shot because though he was totally ignorant of the existence of Murphy, the two were virtual identical twins.
This is shown in the first recorded adventure and explained again in a commentary at the beginning of each subsequent drama with, "A man on the run...Peter Murphy, American agent, a spy has been exposed, who steps into the shoes of his exact double, multi-millionaire Marc Wainwright, when the millionaire accidentally takes the bullet meant for the spy. Thus for Peter Murphy begins a bizarre new life both public and private, a life of double jeopardy as the man who never was."
The adoption of the Wainwright identity by Murphy came about after Wainwright was shot (apparently on one exit from the bar) and Murphy left by another door and, suffering from exhaustion from fleeing his pursuers, passed out within sight of Wainwright's chauffeur. That driver, not knowing the difference, brought him back to his hotel suite where he was put to bed. It is the next morning when Wainwright's wife comes to check on him that the substitution happens.
We discover, along with Murphy, that the relationship between the now dead Wainwright and his wife is less than cordial, undoubtedly due to the deceased's repeated escapades and drunken brawls. This ice-cold attitude is shown in their first "meeting" when his stony attitude, brought about because Murphy has no idea what is going on, is treated by the wife as 'situation normal'. "Must you be such a pig about everything?" she asks him as she walks out of the room disgusted. That likely plays a major part in her decision, once the switch is revealed, to go along with the 'replacement'.
The actual switching idea was not Murphy's; it came from his handler, Colonel Jack Forbes who, examining the body he thought was Murphy only to discover it was Wainwright, tracked the confused agent to Wainwright's hotel and told Murphy he had the best new cover he could ever hope for. "Look, Peter, Mark Wainwright was one of the richest men in the world. He controlled at least a dozen different companies. He could travel anywhere. He knew influential people. It's perfect." When even then Murphy demurred at the idea, Forbes leveled with him saying, "As Peter Murphy you're finished, unless you want to end up behind a desk somewhere in Omaha for the rest of your life."
The continued adventures for Murphy then are two-fold. His position as the faux-Wainwright affords him a lot of privileges and since the man is in charge of an international corporation, his travels all over Europe are easily explained. Therefore, his work with Intelligence is enhanced.
On the downside is the need to pretend to be someone/something he is not. He is not a businessman so acting like one who is good at it will proved challenging. Add to that the fact that though he looks just like Wainwright, he has no knowledge of who his friends are and who his extended family members are. Is he on good footing with them or not? Will a wrong answer here or there give away his deception?
Helping him will be Eva, Mrs. Wainwright, and as she spends more time with Murphy, she begins to find the man that apparently she had always hoped Wainwright would be. Plus she reveals from the beginning that the real Wainwright and his less-than-trustworthy lawyer were up to something very nasty which would devastate her family so she puts it to Murphy quite simply when she asks right after learning her husband was dead, "I don't know who you are or why you're posing as my husband. The most important thing right now is that Mark Wainwright must not be dead. Will you stay ... be my husband!"
His choices at that point are to either go sit behind a desk writing reports in Omaha or be a mega-rich corporate leader married to a gorgeous wife who is begging for his help. It is not hard to guess which one he chooses.